Methods and apparatus for pressure compensation in a mass flow controller

ABSTRACT

Performance of mass flow controller may be vulnerable to pressure transients in a flow path to which the controller is coupled for the purpose of controlling the fluid flow. A system and method are provided for reducing or eliminate performance degradations, instabilities, and/or inaccuracies of a mass flow controller caused by changes in the pressure environment. In particular, a method and system are provided for compensating for pressure transients in the pressure environment of a flow path and mass flow controller.

RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Serial No. 60/397,285, entitled “METHODS ANDAPPARATUS FOR PRESSURE COMPENSATION IN A MASS FLOW CONTROLLER,” filed onJul. 19, 2002, which is herein incorporated by reference in itsentirety.

FIELD OF THE INVENTION

[0002] The present invention relates generally to a method and systemfor controlling the flow rate of a fluid, and more particularly tomethods and systems involving mass flow controllers.

BACKGROUND OF THE INVENTION

[0003] Many industrial processes require precise control of variousprocess fluids. For example, in the pharmaceutical and semiconductorindustries, mass flow controllers are used to precisely measure andcontrol the amount of a process fluid that is introduced to a processchamber. The term fluid is used herein to describe any type of matter inany state that is capable of flow. It is to be understood that the termfluid applies to liquids, gases, and slurries comprising any combinationof matter or substance to which controlled flow may be of interest.

[0004] Conventional mass flow controllers generally include four mainportions: a flow meter, a control valve, a valve actuator, and acontroller. The flow meter measures the mass flow rate of a fluid in aflow path and provides a signal indicative of that flow rate. The flowmeter may include a mass flow sensor and a bypass. The mass flow sensormeasures the mass flow rate of fluid in a sensor conduit that is fluidlycoupled to the bypass. The mass flow rate of fluid in the sensor conduitis approximately proportional to the mass flow rate of fluid flowing inthe bypass, with the sum of the two being the total flow rate throughthe flow path controlled by the mass flow controller. However, it shouldbe appreciated that some mass flow controllers may not employ a bypass,as such, all of the fluid may flow through the sensor conduit.

[0005] In many mass flow controllers, a thermal mass flow sensor is usedthat includes a pair of resistors that are wound about the sensorconduit at spaced apart positions, each having a resistance that varieswith temperature. As fluid flows through the sensor conduit, heat iscarried from the upstream resistor toward the downstream resistor, withthe temperature difference being proportional to the mass flow rate ofthe fluid flowing through the sensor conduit and the bypass.

[0006] A control valve is positioned in the main fluid flow path(typically downstream of the bypass and mass flow sensor) and can becontrolled (e.g., opened or closed) to vary the mass flow rate of fluidflowing through the main fluid flow path, the control being provided bythe mass flow controller. The valve is typically controlled by a valveactuator, examples of which include solenoid actuators, piezoelectricactuators, stepper actuators, etc.

[0007] Control electronics control the position of the control valvebased upon a set point indicative of the mass flow rate of fluid that isdesired to be provided by the mass flow controller, and a flow signalfrom the mass flow sensor indicative of the actual mass flow rate of thefluid flowing in the sensor conduit. Traditional feedback controlmethods such as proportional control, integral control,proportional-integral (PI) control, derivative control,proportional-derivative (PD) control, integral-derivative (ID) control,and proportional-integral-derivative (PID) control are then used tocontrol the flow of fluid in the mass flow controller. In each of theaforementioned feedback control methods, a control signal (e.g., acontrol valve drive signal) is generated based upon an error signal thatis the difference between a set point signal indicative of the desiredmass flow rate of the fluid and a feedback signal that is related to theactual mass flow rate sensed by the mass flow sensor.

[0008] Many conventional mass flow controllers are sensitive tocomponent behavior that may be dependent upon any of a number ofoperating conditions including fluid species, flow rate, inlet and/oroutlet pressure, temperature, etc. In addition, conventional mass flowcontrollers may exhibit certain non-uniformities particular to acombination of components used in the production of the mass flowcontroller which results in inconsistent and undesirable performance ofthe mass flow controller.

[0009] To combat some of these problems, a mass flow controller may betuned and/or calibrated during production. Production generally includesoperating the mass flow controller on a test fluid under a set ofoperating conditions and tuning and/or calibrating the mass flowcontroller so that it exhibits satisfactory behavior.

[0010] As is known to those skilled in the art, the process of tuningand/or calibrating a mass flow controller is an expensive, laborintensive procedure, often requiring one or more skilled operators andspecialized equipment. For example, the mass flow sensor portion of themass flow controller may be tuned by running known amounts of a knownfluid through the sensor portion and adjusting certain filters orcomponents to provide an appropriate response. A bypass may then bemounted to the sensor, and the bypass is tuned with the known fluid toreflect an appropriate percentage of the known fluid flowing in the mainfluid flow path at various known flow rates. The mass flow sensorportion and bypass may then be mated to the control valve and controlelectronics portions and then tuned again, under known conditions.

[0011] When the type of fluid used by an end-user differs from that usedin tuning and/or calibration, or when the operating conditions, such asinlet and outlet pressure, temperature, range of flow rates, etc., usedby the end-user differ from that used in tuning and/or calibration, theoperation of the mass flow controller is generally degraded. For thisreason, additional fluids (termed “surrogate fluids”) and or operatingconditions are often tuned or calibrated, with any changes necessary toprovide a satisfactory response being stored in a lookup table.

[0012] Although the use of additional tuning and/or calibration withdifferent fluids and at different operating conditions can be used toimprove the performance of the mass flow controller, this type ofsurrogate tuning and/or calibration is time consuming and expensive, asthe tuning and/or calibration procedures must be repeated for at leasteach surrogate fluid and likely must be repeated for a number ofdifferent operating conditions with each surrogate fluid. Furthermore,because the surrogate fluids only approximate the behavior of thevarious types of fluids that may be used by the end-user, the actualoperation of the mass flow controller at an end-user site may differsubstantially from that during tuning and/or calibration. Consideringthe wide range of industries and applications employing mass flowcontrollers, the process fluid and operating conditions applied to themass flow controller by an end user are likely to be different than thetest fluids and operating conditions upon which a mass flow controllerwas tuned and/or calibrated, despite tuning and/or calibration of themass flow controller with a number of different surrogate fluids andoperating conditions. Therefore, an apparatus is needed, that isinsensitive to operating conditions and does not require as muchcalibration and/or tuning.

[0013] A flow path to which a mass flow controller is coupled to controlfluid flow may include a portion where the flow meter senses flow, theportion having a bypass and a sensor conduit as described in theforegoing. The flow path is often provided with a pressure regulator tocontrol the pressure at the inlet side of the flow path. Typically, thepressure regulator is provided upstream of the portion of the flow pathto which the flow meter is coupled.

[0014] The pressure regulator maintains a desired inlet pressure of theflow path. Pressure regulators generally do not operate error free andmay introduce pressure transients, or other deviations from the desiredpressure into the flow path. These deviations may have deleteriouseffects on the performance of the mass flow controller. Often the massflow controller must absorb these undesirable pressure transients asbest as possible, and these undesirable transients typically degrade theaccuracy of control and quality of performance of the mass flowcontroller.

SUMMARY OF THE INVENTION

[0015] One aspect of the present invention includes a method in a flowcontroller including a flow sensor coupled to a fluid flow path havingan inlet side and an outlet side, the flow sensor being adapted toprovide a sensor output signal indicative of a sensed fluid flow throughthe flow path, a method comprising acts of measuring at least onepressure of the flow path, and adjusting the sensor output signal basedon the act of measuring the at least one pressure. According to oneembodiment, the method further comprises an act of forming at least onepressure signal based on the at least one pressure. According to oneembodiment, the method further comprises an act of filtering the atleast one pressure signal to provide a false flow signal that emulates aresponse of the flow sensor due to pressure changes in the flow path.According to one embodiment, the method further comprises an act ofadjusting the sensor output includes an act of subtracting the falseflow signal from the sensor output signal.

[0016] Another aspect of the present invention includes a method ofmodifying a sensor output signal from a flow sensor, the methodcomprising acts of constructing a false flow signal corresponding to aresponse of the flow sensor due to changes in pressure based on at leastone pressure measurement of the flow path, and subtracting the falseflow signal from the sensor output signal. According to one embodiment,the method further comprises an act of providing a pressure signalindicative of the at least one pressure measurement. According to oneembodiment, the method further comprises the act of constructing a falseflow signal includes an act of delaying the pressure signal such that itis substantially aligned in time with the sensor output signal.According to one embodiment, the method further comprises the act ofconstructing the false flow signal includes an act of differentiatingthe pressure signal. According to one embodiment, the method furthercomprises an act of constructing the false flow signal includes an actof filtering the pressure signal with at least one filter, the at leastone filter having a transfer function that emulates a response of theflow sensor to the pressure change in the flow path. According to oneembodiment, the at least one filter includes a plurality of 2^(nd)-orderfilters connected in series, and an output from each of the plurality of2^(nd)-order filters are scaled and summed to provide the false flowsignal.

[0017] One aspect of the present invention includes a method of removingfalse flow information from a sensor output signal provided by a flowsensor coupled to a flow path, the false flow information resulting fromthe flow sensor responding to flow changes caused by pressuretransients. The method comprises acts of measuring at least one pressurein the flow path, providing at least one pressure signal indicative ofthe at least one pressure measurement, constructing a false flow signalfrom the at least one pressure signal, and subtracting the false flowsignal from the sensor output signal to provide a flow signal indicativeof the fluid flow in the fluid path.

[0018] One aspect of the present invention includes a method of deadvolume compensation, the method comprising acts of predicting a responseof a sensor to a fluid filling a dead volume due to pressure changes ina fluid flow path, and modifying a sensor output signal provided by thesensor based on the predicted response to essentially remove false flowinformation from the sensor output signal.

[0019] Another aspect of the present invention includes a method ofdetermining a flow rate of a fluid flowing in a conduit, comprising actsof a) sensing a flow rate of the fluid flowing in the conduit, b)measuring a change in pressure of the fluid flowing in the conduit, c)determining an effect of the change in pressure on the flow rate of thefluid sensed by act (a), and modifying the sensed flow rate of the fluidbased upon the effect of the change in pressure to determine the flowrate of the fluid flowing in the conduit.

[0020] Yet another aspect of the present invention relates to a flowmeter comprising a flow sensor adapted to measure fluid flow in a flowpath, the flow sensor providing a sensor output signal in response tosensed fluid flow in the flow path, at least one pressure transducer tomeasure at least one pressure in the flow path, the at least onepressure transducer providing at least one pressure signal related tothe respective at least one measured pressure, a compensation filter toreceive the at least one pressure signal, the compensation filteradapted to construct a false flow signal approximating a response of theflow sensor to pressure transients in the flow path, and a subtractor toreceive the sensor output signal and the false flow signal and toprovide a flow signal related to the difference between the sensoroutput signal and the false flow signal. According to one embodiment ofthe present invention, the compensation filter includes a delay blockthat delays the at least one pressure signal to be substantially alignedin time with the response of the flow sensor to pressure transients, andwherein the delay block provides at least one delayed pressure signal.According to one embodiment of the present invention, the compensationfilter includes a differentiator to receive the delayed pressure signal,the differentiator being adapted to determine a derivative of thedelayed pressure signal to provide a derivative signal.

[0021] Another aspect of the present invention relates to a compensationfilter for generating a false flow signal from a pressure signal, thecompensation filter comprising a differentiator receiving a pressuresignal indicative of a pressure in a fluid path, the differentiatorbeing adapted to determine a derivative of the pressure signal toprovide a derivative signal, and at least one filter having a transferfunction adapted to transform the derivative signal into a false flowsignal indicative of false flow information generated by the flow sensorin response to pressure transients.

[0022] One aspect of the present invention relates to a method ofcompensating for fluid pressure induced changes in the position of thecontrolled portion of a valve, the method comprising acts of measuringat least one pressure in a valve environment, providing at least onepressure signal indicative of the at least one pressure measurement,respectively, calculating a displacement of the controlled portion ofthe valve based on the at least one pressure signal, and generating acompensation drive level to move the controlled portion of the valve anamount having an opposite sign of and substantially equal in magnitudeto the calculated displacement.

[0023] Another aspect of the present invention includes a method ofpreventing the movement of the controlled portion of the a valve due topressure transients, the method comprising acts of predicting adisplacement a pressure transient will force the controlled portion of avalve to move based on at least one pressure measurement of a valveenvironment, and moving the controlled portion of the valve tocounter-act the predicted displacement.

[0024] One aspect of the present invention includes an apparatus coupledto a flow path, the apparatus comprises a pressure measurement device tomeasure at least one pressure in a flow path environment and to provideat least one pressure signal indicative of the at least one measuredpressure, and displacement compensation means for receiving the at leastone pressure signal and for providing a displacement compensation signalindicating a drive level to compensate for valve displacement of a valvecoupled to the flow path caused by pressure changes in the flow pathenvironment.

[0025] According to one embodiment, the displacement compensation meanscomprises means for calculating the displacement compensation signalbased on a force valve model. According to another embodiment, the forcevalve model includes a magnetic model of the valve. According to oneembodiment, the force valve model has a parameter that indicates apressure gradient in the valve environment.

[0026] According to one aspect of the invention, a flow meter isprovided comprising a flow sensor adapted to sense fluid flow in a fluidflow path and to provide a sensor output signal indicative of the sensedfluid flow, at least one pressure transducer adapted to measure at leastone pressure in a fluid flow path environment and to provide at leastone pressure signal indicative of the at least one measured pressure,and a compensation filter to receive the at least one pressure signaland to construct a false flow signal related to the at least onepressure signal.

[0027] According to one embodiment, the false flow signal is constructedto recreate false flow information resulting from the flow sensorresponse to flow fluctuations caused by pressure transients in the flowpath. According to another embodiment, the compensation filter includesa transfer function that emulates a response of the flow sensor topressure transients in the flow path. According to one embodiment, thefalse flow signal is subtracted from the sensor output signal to providea flow signal. According to another aspect of the invention, in a massflow controller coupled to a flow path, the mass flow controller havinga control loop including a flow meter, a controller, a valve actuatorand a valve, a method is provided comprising acts of measuring at leastone pressure in a fluid path environment, providing at least onepressure signal indicating at least one pressure measurement,determining at least one compensation signal based on at least onepressure measurement, and applying the at least one compensation signalto the control loop of the mass flow controller.

[0028] According to one embodiment, the method further comprises an actof determining at least one compensation filter includes constructing afalse flow signal to recreate false flow information resulting from aresponse of the flow meters to pressure transients in the flow pathenvironment. According to another embodiment, the method furthercomprises an act of applying the at least one compensation signal to thecontrol loop includes an act of applying the false flow signal to thecontrol loop to compensate for the flow meters response fluctuations influid flow due to pressure transients in the flow path. According to afurther embodiment, the method further comprises an act of determiningthe at least one compensation signal includes determining a displacementcompensation signal indicative to a drive level to compensate for avalve displacement due to pressure transients. According to oneembodiment, the method further comprises an act of determining the atleast one compensation signal includes determining a false flow signaland a displacement compensation signal.

[0029] According to yet another aspect of the invention, a mass flowcontroller is provided comprising a flow meter adapted to sense fluidflow in a fluid flow path and provide a flow signal indicative of themass flow rate in the flow path, a controller coupled to the flow meterand adapted to provide a drive signal based at least in part on the flowsignal, a valve actuator adapted to receive the drive signal from thecontroller, a valve adapted to be controlled by the valve actuator andcoupled to the fluid flow path, at least one pressure transducer tomeasure at least one pressure in a mass flow controller environment andto provide at least one pressure signal indicative of measurement of theat least one pressure, and at least one compensation means to receive atleast one pressure signal and to provide at least one compensationsignal to the control loop to compensate for effects of a pressurechanges in the mass flow controller environment, wherein the controlloop of the mass flow controller includes the flow meter, thecontroller, the valve actuator, and the valve.

[0030] According to one embodiment, the at least one transducer measuresan inlet pressure of the flow path and provides an inlet pressuresignal. According to one embodiment, the at least one compensation meansincludes a compensation filter to receive the inlet pressure signal andto construct a false flow signal from the inlet pressure signal.According to another embodiment, the flow meter includes a flow sensoradapted to sense fluid flow in the flow path and adapted to provide asensor output signal indicative of the sensed fluid flow. According toanother embodiment, the compensation filter has a transfer function thatemulates the response of the flow sensor to fluid flow resulting fromchanges in inlet pressure.

[0031] According to another embodiment, the false flow signal isconstructed to recreate a false flow information component of the sensoroutput signal resulting from changes in inlet pressure. According to oneembodiment, the flow signal is determined by subtracting the false flowsignal from the sensor output signal. According to one embodiment, thecompensation means includes displacement compensation means thatreceives the inlet pressure signal and provides a displacementcompensation signal indicative of a drive level to maintain a controlledportion of the valve substantially motionless in a pressure environmentof the valve. According to one embodiment, the displacement compensationsignal is added to the drive signal to compensate for valve displacementresulting from pressure gradients in the pressure environment of thevalve. According to one embodiment, the displacement compensation signalis based in part on a force model of the valve. According to oneembodiment, the force model of the valve includes a magnetic model ofthe valve.

[0032] According to one embodiment, the force model of the valveincludes a parameter for at least one pressure drop across the valve.According to one embodiment, the compensation means includes acompensation filter receiving at least one pressure signal and providinga false flow signal constructed to recreate false flow informationresulting from the flow meter responding to pressure transients anddisplacement compensation means to receive at least one pressure signaland to provide a displacement compensation signal indicative of a drivelevel to compensate for valve displacement caused by a pressure change.

[0033] One aspect of the present invention includes a method ofconfiguring a mass flow controller for operation with process operatingconditions that differ at least in part from test operating conditionsused during production of the mass flow controller, the method comprisesacts of establishing a response of the mass flow controller with thetest operating conditions, and modifying at least one control parameterof the mass flow controller based on the process operating conditionssuch that the response of the mass flow controller operating with theprocess operating conditions does not substantially change.

[0034] According to one embodiment, the method further comprises the actof modifying the at least one control parameter includes an act ofdetermining a plurality of process gain terms associated with aplurality of components of the mass flow controller based on the processoperating conditions, the plurality of components forming a control loopof the mass flow controller. According to one embodiment, the methodfurther comprises the act of determining the plurality of process gainterms includes an act of determining a process reciprocal gain termformed by taking a reciprocal of a product of the plurality of processgain terms, the process reciprocal gain term being a function of atleast one variable operating condition. According to one embodiment, themethod further comprises at least one variable operating conditionincludes at least one pressure in the mass flow controller environment.According to one embodiment, the method further comprises at least onevariable operating condition includes an inlet pressure. According toone embodiment, the method further comprises at least one variableoperating condition includes a set point.

[0035] One aspect of the present invention includes a computer readablemedium encoded with a program for execution on a processor, the program,when executed on the processor performing a method of configuring a massflow controller for operation with a set of process operating conditionsthat differ at least in part from a set of test operating conditionsused to establish a response of the mass flow controller duringproduction, the method comprises acts of receiving as an input at leastone of process fluid species information and process operatingconditions, and modifying at least one control parameter of the massflow controller based on the input such that the response of the massflow controller does not substantially change when operated with theprocess operating conditions.

[0036] According to one embodiment, that act of modifying the at leastone control parameter includes an act of determining a plurality ofprocess gain terms associated with a plurality of components of the massflow controller operating with the process operating conditions, theplurality of components forming a control loop of the mass flowcontroller. According to one embodiment, the act of determining theplurality of gain terms includes an act of determining a processreciprocal gain term formed by taking a reciprocal of a product of theplurality of gain terms, the process reciprocal gain term being afunction of at least one variable operating condition. According to oneembodiment, the at least one variable operating condition includes atleast one pressure in the mass flow controller environment. According toone embodiment, the at least one variable operating condition includesan inlet pressure. According to one embodiment, at least one variableoperating condition includes a set point.

[0037] In another aspect of the invention, a mass flow controller isprovided having a control loop. The mass flow controller comprises aflow meter adapted to sense fluid flow in a fluid flow path and providea flow signal indicative of the mass flow rate in the flow path, acontroller coupled to the flow meter and adapted to provide a drivesignal based at least in part on the flow signal, a valve actuatoradapted to receive the drive signal from the controller, a valve adaptedto be controlled by the valve actuator and coupled to the fluid flowpath, wherein the control loop of the mass flow controller includes theflow meter, the controller, the valve actuator, and the valve, andwherein the control loop is adapted to have a substantially constantcontrol loop gain term with respect to at least one variable operatingcondition during operation.

[0038] According to one embodiment, the at least one variable operatingcondition includes at least one pressure in the mass flow controllerenvironment. According to one embodiment, the at least one variableoperating condition includes an inlet pressure. According to oneembodiment, the at least one variable operating condition includes a setpoint.

[0039] According to another aspect of the invention, a compensationfilter is provided for generating a false flow signal from a pressuresignal. The compensation filter comprises a plurality of filters, atleast two of which are connected in series, and wherein a respectiveoutput of each of the at least two filters are scaled and summed. In oneembodiment of the invention, the compensation filter further comprises adifferentiator that is adapted to differentiate the pressure signal, andwhich provides a differentiated signal to the plurality of filters.According to another embodiment, the compensation filter furthercomprises a delay that delays the pressure signal, and which provides adelayed pressure signal to the plurality of filters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In the drawings:

[0041]FIG. 1 illustrates a schematic block diagram of an example massflow controller with which various aspects of the present invention maybe implemented;

[0042]FIG. 2 is a more detailed schematic block diagram of the flowmeter shown in FIG. 1;

[0043]FIG. 3 illustrates various output signals of a mass flow sensor inresponse to a step change in flow according to an embodiment of thepresent invention;

[0044]FIG. 4 is a more detailed schematic diagram of the Gain/Lead/Lagcontroller circuit shown in FIG. 1;

[0045]FIG. 5 is a more detailed schematic block diagram of the valveactuator shown in FIG. 1;

[0046]FIG. 6 illustrates signal waveforms of a number of the signalsshown in FIG. 4;

[0047]FIGS. 7a-7 f illustrates a method of configuring a mass flowsensor for operation with a process fluid and/or process operatingconditions according to an embodiment of the present invention;

[0048]FIG. 8 illustrates a compensation filter according to oneembodiment of the present invention;

[0049]FIG. 9 illustrates one method of pressure induced valvedisplacement compensation according to one embodiment of the invention;

[0050]FIG. 10 illustrates a free floating plunger;

[0051]FIG. 11A is a graph of a pressure pulse introduced at an inletside of a flow path as a function of time;

[0052]FIG. 11B is a graph of a pressure signal resulting from thepressure pulse shown in FIG. 1A;

[0053]FIG. 11C illustrates a flow path having a pressure tranducer todetect pressure changes according to one embodiment of the invention;

[0054]FIG. 11D shows a compensation filter that compensates for falseflow information according to one embodiment of the invention;

[0055]FIG. 12A illustrates a case in which a pressure transient in theshape of a pressure pulse is introduced at an inlet side of a flow path;

[0056]FIG. 12B shows a sensor output as a function of time resultingfrom the pressure transient shown in FIG. 12A;

[0057]FIG. 13 illustrates one method of pressure induced valvedisplacement compensation according to one embodiment of the invention;

[0058]FIG. 14 illustrates a system that facilitates automaticconfiguration of a mass flow controller according to one embodiment ofthe invention;

[0059]FIG. 15 illustrates another system that facilitates automaticconfiguration of a mass flow controller according to one embodiment ofthe invention; and

[0060]FIG. 16 illustrates a cross-sectional view of a valve.

DETAILED DESCRIPTION OF THE INVENTION

[0061] This application contains subject matter that is related to U.S.patent application Ser. No. 10/131,603, entitled SYSTEM AND METHOD FOR AMASS FLOW CONTROLLER, filed Apr. 24, 2002, which is herein incorporatedby reference in its entirety.

[0062] Typically a fluid flow path exists in a pressure environment. Thepressure environment may include the pressure at the inlet side of theflow path (referred to as inlet pressure), and pressure at the outletside of the valve (referred to as outlet pressure), and other pressureswithin the environment. For example, the pressure environment of theflow path may also include pressure differentials such as, for example,the pressure drop across a bypass or across a valve. The pressureenvironment may also include various pressure transients includingpulses introduced by a regulator, turbulence caused by the geometry of aflow sensor, or various other pressure perturbations. However, thepressure environment is not often monitored. As such, performance ofmass flow controller may be vulnerable to pressure transients in a flowpath to which the controller is coupled for the purpose of controllingthe fluid flow.

[0063] According to one aspect of the present invention, Applicants haverecognized that measurements of the pressure environment of a flow pathmay be used to reduce or eliminate performance degradations,instabilities, and/or inaccuracies of a mass flow controller caused bychanges in the pressure environment. As such, Applicants have developedvarious methods for compensating for pressure transients in the pressureenvironment of a flow path and mass flow controller.

[0064] As discussed in the foregoing, a mass flow controller typicallyincludes a flow meter that senses the fluid flow in a fluid flow path.The flow rate sensed by the flow meter is often part of a feedbackcontrol loop that controls the flow rate of a fluid being provided to aprocess (e.g., a semiconductor fabrication process) at the outlet sideof the flow path.

[0065] In many cases, the actual flow rate provided to the process mustbe accurately controlled. However, the pressure transients may causelocal fluctuations in the fluid flow that are sensed by the flow meter.These local fluctuations may not be an accurate indication of the actualflow rate being provided to the process. This false flow information isthen provided to the control loop of a mass flow controller. Thecontroller may then adjust the flow rate provided to the process inresponse to the false flow information. As such, the controller maymomentarily lose control of the process and/or provide undesired flowrates to the process.

[0066] As used herein, the term false flow refers to fluid flow thatdoes not correspond to the actual flow provided to a process. Forexample, local variations or fluctuations in fluid flow that are notsubstantially experienced at the outlet side of a flow path isconsidered false flow. As such, false flow information generallydescribes an indication of flow that does not correspond to the flowrate being provided to a process.

[0067] According to one embodiment of the invention, it is appreciatedthat may be advantageous to measure the pressure of the flow path (e.g.,the inlet pressure) and provide a control system that incorporates thisinformation. More particularly, to reduce the performance degradationdue to pressure transients, it may be desirable to measure the pressurein the flow path and adjust control parameters of a mass flow controllerin response to the changes in pressure.

[0068] One embodiment of the present invention includes measuring theinlet pressure of a flow path and providing the inlet pressuremeasurement to the mass flow controller. For instance, a pressuretransducer may be coupled to the flow path to provide a pressure signalindicative of the inlet pressure of the flow path.

[0069] Applicants have recognized and appreciated that by providing apressure signal to a mass flow controller, various deficiencies in theconventional operation of the mass flow controller can be addressed.Accordingly, applicants have identified various methods of utilizing apressure signal to improve the performance and accuracy of a mass flowcontroller. One method according to one embodiment of the presentinvention includes compensating for spurious flow signals that may occurdue to pressure transients in a fluid path coupled to a mass flowcontroller.

[0070] One problem associated with pressure transients in a flow paththat may have deleterious effects on a mass flow controller is describedbelow. When the pressure in a flow path changes, fluid accelerates downthe pressure gradient in order to fill the volume, referred to as deadvolume, created by the change in pressure. A sensor of a flow meter mayregister an increase in fluid flow due to this local acceleration of thefluid into the dead volume. However, this is considered a false flow offluid because this flow is not indicative of the flow being provided tothe process. As such, the sensor output signal from the sensor carriesfalse flow information that is propagated to the control loop of themass flow controller. As discussed above, this false flow informationmay have undesirable consequences with respect to the accuracy andperformance of the mass flow controller.

[0071]FIG. 12A illustrates a case in which a pressure transient in theshape of a pressure pulse is introduced at an inlet side of a flow path.Graph 1200 a shows a plot of a pressure pulse 1210 as a function oftime. Pressure pulse 1210 is introduced to the inlet side of the flowpath. As a result, the flow sensor responds with sensor output signal1220 as shown in graph 1200 b of FIG. 12B. Assuming that the actual flow(i.e., the flow being delivered to the process) of the flow path has notchanged, the spike in the sensor output signal contains a large falseflow component with respect to the actual flow. As such, the mass flowcontroller reacts to the flow spike accordingly and may momentarily losecontrol of the process.

[0072] According to one embodiment of the present invention, applicantshave recognized that pressure measurements in the flow path may beutilized to anticipate false flow indications and compensate for thenegative impact they may incur on a mass flow controller. One embodimentof the present invention includes a method for controlling flowincluding measuring the pressure in a fluid flow path and adjusting anoutput signal provided by a flow sensor coupled to the flow path basedon the pressure measurements.

[0073] By analyzing a flow sensor's response fluid flow fluctuationscaused by pressure transients, Applicants have developed methods forgenerating a false flow signal that recreates the false flow componentof a flow signal provided by a flow sensor in response to a pressuretransient. This generated false flow signal may be used by a system tocompensate for the spike in flow sensor output. For instance, thissignal may be used by a compensator to reduce induced value drive motionresulting from the spike in output.

[0074] FIGS. 11A-D illustrate one embodiment of the invention forgenerating a false flow signal from a pressure signal indicative of thepressure measured in a fluid flow path. The false flow signal can thenbe subtracted from the flow signal provided by the flow sensor toproduce an indicated flow signal that does not include the false flowinformation. As such, the false flow information is prevented fromcausing the controller to respond erroneously (e.g., by providingunwanted flow to the process).

[0075] Graph 1100 a shown in FIG. 11A illustrates a pressure transient,and in particular, a pressure pulse 1110, that a flow path mayexperience during operation. Graph 1100 b of FIG. 11B shows a pressuresignal 1120 resulting from pressure pulse 1110. The pressure signal maybe measured by a pressure measurement device (e.g., a pressuretransducer) coupled to the flow path and adapted to measure the pressureat some portion of the flow path.

[0076]FIG. 11C shows a system 1100 c having a flow path 200 with a flowsensor 1140 being coupled to flow path 200 to sense fluid flow in flowpath 200. As shown, pressure pulse 1100 is introduced to the flow path200 (e.g. by non-ideal performance of an upstream regulator) at theinlet of flow path 200. Pressure pulse 1100 may cause a localfluctuation in the fluid flow sensed by flow sensor 1140. Flow sensor1140, in turn, produces a sensor output signal 1150 that is corruptedwith false flow information.

[0077] According to one aspect of the present invention, a compensationfilter is provided that compensates for the false flow information. Inone embodiment of the invention, as shown in FIG. 11D, a compensationfilter 1180 is provided that receives the pressure signal 1120 producedby transducer 295 and produces a false flow signal 1160. Because filter1180 receives pressure information indicative of the pressure in aportion of the flow path, compensation filter 1180 can predict theresponse the flow sensor will have to the fluid flow fluctuationsresulting from the pressure transient.

[0078] As such, filter 1180 may construct a false flow signal thatclosely resembles the false flow information produced by the sensor.More particularly, filter 1180 recreates the false flow informationproduced by flow sensor 1140 and provides this information as false flowsignal 1160. False flow signal 1160 can then be subtracted from thesensor output signal 1150 to effectively remove the effects of pressurepulse 1100. In one embodiment, a false flow signal includes a transferfunction that emulates the behavior of the flow sensor in response toflow fluctuations caused by pressure transients.

[0079] According to one embodiment of the present invention, acompensation filter 800 is provided that emulates the behavior of theflow sensor. More particularly, FIG. 8 shows a compensation filter 800that includes a time delay block 810, a differentiator 820, aseries-connected bank of 2^(nd)-order filters 830 a-f (collectively,item 830), and an adder 840. Compensation filter 800 receives pressuresignal 1120 and provides it to time delay block 810. Time delay block810 delays the pressure signal such that the delayed output signal isaligned in time with a sensor output signal (not shown). In particular,some finite amount of time elapses between a pressure transient and whena flow sensor responds to the pressure transient (i.e., there is a delaybetween a pressure pulse and when the false flow information appears onthe sensor output signal). As such, the pressure signal may be delayedsuch that the generated false flow signal is subtracted off the properportion of the sensor output signal.

[0080] Delay block 810 provides a delayed pressure signal 815 todifferentiator 820 which calculates a derivative of the delayed pressuresignal 815 and provides a derivative signal 825 to a series ofsecond-order filters 830. The derivative of the delayed pressure signalis calculated because the false flow resulting from a pressure transientis proportional to the pressure gradient resulting from a pressuretransient. In addition, the derivative of the delayed pressure signalensures that a constant pressure results in a zero false flow signal.That is, when the pressure signal is constant, the compensation filterhas no effect on the sensor output signal.

[0081] Derivative signal 825 is provided to the first second orderfilter 830 a in the series of filters 830. The output of each secondorder filter is provided as the input to the next second order filter inseries 830. In addition, the output from each second-order filter istapped off and provided to a respective gain block 850 a-850 f thatscales the respective output of each filter by a respective constantgain factor K_(n).

[0082] Each of the scaled outputs from the individual 2^(nd)-orderfilters contributes to the construction of false flow signal 1160. Adder840 sums the contributions of the scaled outputs and provides the falseflow signal 1160. In one embodiment, false flow signal 1160 is arecreation of the false flow information provided by a flow sensor inresponse to pressure transients. As such, false flow signal 1160 may besubtracted from the sensor output to compensate for this false flowinformation.

[0083] It should be appreciated that the number of filters and type offilters illustrated in FIG. 8 is not limiting. Indeed, any filterconfiguration of any order and arrangement may be used to provide afalse flow signal. The configuration illustrated in FIG. 8 has beenshown to provide sufficient control over characteristics of the falseflow signal that Applicants have found useful such as dead time, risetime, overshoot and parabolic attributes such that a false flow signalclosely resembling the false flow information superimposed on the sensoroutput signal in response to a pressure transient may be recreated.However, other filter designs and arrangements that will occur to thoseskilled in the art may be applicable and are considered to be within thescope of the present invention. For instance, the order of severalcomponents may be different (e.g., delay block 810, differentiator 820),and/or one or more of these blocks may be eliminated altogether.

[0084] In one example, design of one embodiment of the filters shown inFIG. 8 are described in more detail below. A generalized second ordertransfer function of the second order filters can be represented as:

K/(s ²+2ξω_(n)s+ω_(n) ²)  (1)

[0085] Where:

[0086] K=Gain

[0087] s=Laplace Operator

[0088] ω_(n)=Natural Frequency

[0089] ξ=Damping Factor

[0090] Scaling factors may be added such that each filter can betailored independent of each other. As such, the filter bank 830 may beoptimized to provide a different response in terms of “height” (gain),“width” (frequency response) and over/undershoot (damping) such that theshape of the constructed false flow signal can be “dialed” and bychanging the scaling factors ξ, ω, and δ.

[0091] One exemplary specific transfer function can be represented as:

Kδω _(n) ²/(s ²+ξδω_(n) s+δ ²ω_(n) ²)  (2)

[0092] The K term in the transfer function is illustrated as a constantgain factor K_(n). As such, the output from each second order filter ismultiplied by K_(n) and provided to adder 840. Adder 840 sums thecontributions from each filter to provide false flow signal 1160. Falseflow signal 1160 is subtracted from the sensor output signal to providean indicated flow signal. As such, the false flow informationsuperimposed on the flow signal due to pressure transients is subtractedoff by the constructed false flow signal leaving a flow signalindicative of the actual flow supplied to the process at the outlet sideof the flow path.

[0093] Mass flow controllers are often vulnerable to instability due tofactors including non-linearities in the various components of the massflow controller dependencies on various operating conditions of a massflow controller, or other factors. The term operating condition appliesgenerally to any of various conditions that can be controlled and thatmay influence the operation of a mass flow controller. In particular,operating conditions apply to various external conditions that can becontrolled independent of a particular mass flow controller. Exemplaryoperating conditions include, but are not limited to, fluid species, setpoint or flow rate, inlet and/or outlet pressure, temperature, etc.

[0094] However, it should be appreciated that other internal conditionsmay be present during the operation of a mass flow controller such assignal characteristics, system noise, or perturbations that cannot becontrolled independent of a particular flow controller. In particular,various signals employed by the mass flow controller may have frequencycomponents containing many different frequencies. However, the frequencycomposition of a signal is inherent to the signal and is not consideredto be controllable independent of a particular mass flow controller.Accordingly, such conditions, unless specifically stated otherwise, arenot considered to be encompassed within the term operating conditions asused herein.

[0095] The term mass flow rate, fluid flow, and flow rate is usedinterchangeably herein to describe the amount of fluid flowing through aunit volume of a flow path (e.g. flow path 103 of FIG. 1), or a portionof the flow path, per unit time (i.e., fluid mass flux).

[0096] The term species applies generally to the properties of aspecific instance of a fluid. A change in species applies to a change inat least one property of a fluid that may change or affect theperformance of a mass flow controller. For example, a change in speciesmay include a change in fluid type (e.g., from nitrogen to hydrogen), achange in the composition of a fluid (e.g., if the fluid is acombination of gases or liquids, etc.), and/or a change in the state ofthe fluid or combination of fluids. The term species information appliesgenerally to any number of properties that define a particular fluidspecies. For example, species information may include, but is notlimited to, fluid type (e.g. hydrogen, nitrogen, etc.), fluidcomposition (e.g., hydrogen and nitrogen), molecular weight, specificheat, state (e.g., liquid, gas, etc.), viscosity, etc.

[0097] Often a mass flow controller comprises several differentcomponents (i.e., a flow sensor, feedback controller, valve etc.)coupled together in a control loop. Each component that is part of thecontrol loop may have an associated gain. In general, the term gainrefers to the relationship between an input and an output of aparticular component or group of components. For instance, a gain mayrepresent a ratio of a change in output to a change in input. A gain maybe a function of one or more variables, for example, one or moreoperating conditions and/or characteristics of a mass flow controller(e.g., flow rate, inlet and/or outlet pressure, temperature, valvedisplacement, etc.) In general, such a gain function is referred toherein as a gain term. A gain term, and more particularly, therepresentation of a gain term may be a curve, a sample of a function,discrete data points, point pairs, a constant, etc.

[0098] Each of the various components or group of components of a massflow controller may have an associated gain term. A component having noappreciable gain term can be considered as having a unity gain term.Relationships between gain terms associated with the various componentsof a mass flow controller is often complex. For example, the differentgain terms may be functions of different variables (i.e., operatingconditions and/or characteristics of the components), may be in partnon-linear, and may be disproportionate with respect to one another.

[0099] Accordingly, the contributions of each gain term associated withthe components around a control loop of a mass flow controller is itselfa gain term. This composite gain term may itself be a function of one ormore variables and may contribute, at least in part, to the sensitivityof the mass flow controller with respect to change in operatingconditions and/or characteristics of the various components of the massflow controller. According to one embodiment of the present invention, amass flow controller is provided having a control loop with a constantloop gain. According to one embodiment, the constant loop gain isprovided by determining a reciprocal gain term by forming the reciprocalof the product of the gain terms associated with one or more componentsin the control loop of the mass flow controller and applying thereciprocal gain term to the control loop. According to one embodiment,the pressure signal is used to adjust the gain in the mass flowcontroller (e.g., in a GLL controller associated with the mass flowcontroller) to provide a constant gain.

[0100] A constant loop gain as used herein describes a gain of a controlloop of a mass flow controller that remains substantially constant withrespect to one or more operating conditions of the mass flow controller.In particular, a constant loop gain does not vary as a function ofspecific operating conditions associated with a mass flow controller, oras a function of the individual gain terms associated with the controlloop. It should be appreciated that a constant loop gain may not beprecisely constant. Imprecision in measurements, computation andcalculations may cause the constant loop gain to vary. However, suchvariation should be considered encompassed by the definition of aconstant loop gain as used herein. Further, a constant loop gain may notnecessarily be constant over all operating ranges or conditions.However, one benefit of having a constant loop gain over operatingconditions includes the mass flow controller being able to operate (andbe tuned and calibrated) for one fluid and not need to be tuned and/orcalibrated for other fluids and/or operating conditions.

[0101] It should further be appreciated that the gain of certaincomponents of the mass flow control may vary with operating frequency,and that signals of the mass flow controller may have frequencycomponents at many different frequencies. However, frequency is notconsidered an operating condition, and as such, is not considered as acondition over which a constant loop gain remains constant.

[0102] Following below are more detailed descriptions of variousconcepts related to, and embodiments of, methods and apparatus accordingto the present invention for control and configuration of a mass flowcontroller. Such a flow controller with which various aspects may beimplemented is described with particularity in U.S. patent applicationSer. No. 10/131,603, entitled SYSTEM AND METHOD FOR A MASS FLOWCONTROLLER, filed Apr. 24, 2002, incorporated by reference herein in itsentirety. Although various aspects of the present invention may beimplemented in the mass flow controller described therein, it should beappreciated that any mass flow controller may be used, and the inventionis not limited to being implemented in any particular mass flowcontroller.

[0103] It should also be appreciated that various aspects of theinvention, as discussed above and outlined further below, may beimplemented in any of numerous ways, as the invention is not limited toany particular implementation. Examples of specific implementations areprovided for illustrative purposes only.

[0104] In the following description, various aspects and features of thepresent invention will be described. The various aspects and featuresare discussed separately for clarity. One skilled in the art willappreciate that the features may be selectively combined in a mass flowcontroller depending on the particular application.

[0105] A. Control of a Mass Flow Controller

[0106]FIG. 1 illustrates a schematic block diagram of a mass flowcontroller according to one embodiment of the present invention. Themass flow controller illustrated in FIG. 1 includes a flow meter 110, aGain/Lead/Lag (GLL) controller 150, a valve actuator 160, and a valve170.

[0107] The flow meter 110 is coupled to a flow path 103. The flow meter110 senses the flow rate of a fluid in the flow path, or portion of theflow path, and provides a flow signal FS2 indicative of the sensed flowrate. The flow signal FS2 is provided to a first input of GLL controller150.

[0108] In addition, GLL controller 150 includes a second input toreceive a set point signal S12. A set point refers to an indication ofthe desired fluid flow to be provided by the mass flow controller 100.As shown in FIG. 1, the set point signal SI2 may first be passed througha slew rate limiter or filter 130 prior to being provided to the GLLcontroller 150. Filter 130 serves to limit instantaneous changes in theset point in signal SI1 from being provided directly to the GLLcontroller 150, such that changes in the flow take place over aspecified period of time. It should be appreciated that the use of aslew rate limiter or filter 130 is not necessary to practice theinvention, and may be omitted in certain embodiments of the presentinvention, and that any of a variety of signals capable of providingindication of the desired fluid flow is considered a suitable set pointsignal. The term set point, without reference to a particular signal,describes a value that represents a desired fluid flow. Based in part onthe flow signal FS2 and the set point signal SI2, the GLL controller 150provides a drive signal DS to the valve actuator 160 that controls thevalve 170. The valve 170 is typically positioned downstream from theflow meter 110 and permits a certain mass flow rate depending, at leastin part, upon the displacement of a controlled portion of the valve. Thecontrolled portion of the valve may be a moveable plunger placed acrossa cross-section of the flow path, as described in more detail withrespect to FIG. 16. The valve controls the flow rate in the fluid pathby increasing or decreasing the area of an opening in the cross sectionwhere fluid is permitted to flow. Typically, mass flow rate iscontrolled by mechanically displacing the controlled portion of thevalve by a desired amount. The term displacement is used generally todescribe the variable of a valve on which mass flow rate is, at least inpart, dependent. As such, the area of the opening in the cross sectionis related to the displacement of the controlled portion, referred togenerally as valve displacement.

[0109] The displacement of the valve is often controlled by a valveactuator, such as a solenoid actuator, a piezoelectric actuator, astepper actuator etc. In FIG. 1, valve actuator 160 is a solenoid typeactuator, however, the present invention is not so limited, as otheralternative types of valve actuators may be used. The valve actuator 160receives drive signal DS from the controller and converts the signal DSinto a mechanical displacement of the controlled portion of the valve.Ideally, valve displacement is purely a function of the drive signal.However, in practice, there may be other variables that affect theposition of the controlled portion of the valve.

[0110] For example, in the valve illustrated in FIG. 10, a pressuredifferential between the backside of the plunger 1026 and the face ofthe plunger 1025, over the jet orifice 1040 and plateau 1050 attempts toforce the plunger towards the jet. The plunger face over the orificeexperiences a pressure substantially equal to the outlet pressure of theflow path. From the edge of the orifice 1045 to the outer edge of theplateau 1055, the plunger face experiences a pressure gradient, withpressure at the outer edge of the plateau substantially equal to theinlet pressure less any pressure drop through the sensor bypass. Theremainder of the plunger, including the backside, experiences a pressuresubstantially equal to the inlet pressure less any pressure drop throughthe sensor bypass. Accordingly, the plunger 1020 will experience apressure dependent force that can be expressed as: Force=(P₁−P₀)*A,where, P₁ is equal to the inlet pressure, P₀ is equal to the outletpressure, and A is equal to the effective area of the plunger. Theeffective area of the plunger may change from valve to valve istypically within the range of the area of the orifice and the area ofthe orifice plus the plateau.

[0111] As such, when the valve experiences a pressure transient, thisforce changes and the plunger may undergo undesirable displacement. Thatis the plunger may be displaced by some amount different than the valvedisplacement that is desired by the control loop. This undesirabledisplacement may provide a fluid flow to the process having a componentthat is unintended. In addition, this undesired displacement may causethe control loop to oscillate as described below.

[0112] However, if pressure transients that may cause undesirablemovement of the controlled portion of the valve can be detected, thenthe drive signal applied to the valve actuator can be adjusted tocompensate for this undesired valve displacement. Stated differently,the drive signal may be adjusted such that it has a component indicativeof the drive level necessary to keep the plunger stationary under adetected pressure transient.

[0113] Accordingly, one embodiment according to the present inventionincludes determining a displacement compensation signal from a pressuremeasurement, wherein the displacement compensation signal is the drivelevel necessary to prevent the plunger from moving due to pressuretransients. The displacement compensation signal is then added to thevalve drive signal. As such, the valve drive signal applied to the valvehas a component indicating the valve displacement desired by the controlloop of the mass flow controller and a component indicating the drivelevel necessary to hold the plunger steady in the pressure environmentrecorded by the pressure measurements.

[0114] The term pressure environment refers generally to variouspressures that a valve experiences. As the different portions of thevalve may “see” different pressures and at different times, the termpressure environment is meant to refer to the entire set of pressuresthat may affect a force on the valve. Similarly, a valve environmentrefers to the set of forces that act on the valve and may includepressures, magnetic forces, spring forces, mechanical forces etc., asdescribed in further detail below.

[0115] One embodiment according to the present invention involves usinga force model of the valve to predict the pressure induced valvedisplacement from a pressure signal indicative of at least one pressuremeasurement in the valve environment.

[0116]FIG. 9 illustrates one method of pressure induced valvedisplacement compensation. FIG. 9 illustrates the outlet side of a flowpath 200. Valve 170 is coupled to the flow path to control the fluidflow through the outlet to a process. Valve actuator 160 controls thedisplacement of the valve depending on the drive level indicated bydrive signal DS′. For example, valve and valve actuator pair 170 and 160may be the same as that described in connection with FIG. 1.

[0117] In addition, a pressure transducer 295′ is coupled to the flowpath. The pressure transducer measures at least one pressure in thevalve environment. The pressure transducer 295′ provides at least onepressure signal indicative of at least one pressure in the valveenvironment (e.g., inlet pressure, outlet pressure, etc.). For thepurpose of this example, the pressure transducer measures the inletpressure of the flow path and provides pressure signal 270″ indicatingthe inlet pressure. While the pressure transducer is illustrated asbeing upstream from the valve, it should be appreciated that it may beplaced downstream of the valve. In addition, more than one pressuretransducer may be disposed along the flow path in order to measure anydesirable pressure in the valve environment and to output an associatedpressure signal indicative of the pressure measurement.

[0118] Pressure signal 270″ is provided to displacement compensationblock 920. displacement compensation determines a drive level sufficientto substantially counter-act a pressure induced displacement effected onthe valve by the pressure environment indicated by pressure signal 270″.Displacement compensation block 920 provides displacement compensationsignal to summation block 950. At summation block 950, the displacementcompensation signal is added to the drive signal DS issued from acontroller 150. For example, controller 150 may be a GLL controller asillustrated in FIG. 1. The summed drive signal DS′ is then provided tothe valve actuator which mechanically displaces the controlled portionof the valve according to drive signal DS′.

[0119] As such, drive signal DS′ has a component that effectively zeroesout the force effect the pressure environment has on the valvedisplacement and a component provided by the control loop. As such, thenet valve displacement resulting from the valve environment is thedisplacement desired by the control loop of the mass flow controller.

[0120] In one embodiment of displacement compensation, a force model ofa valve is used in order to determined the pressure induced displacementof the valve in a pressure environment. FIG. 13 is similar to FIG. 9,however, the displacement compensation 920′ includes a force model 1300that models the forces in the valve environment. On suitable force modelfor a valve operating with a free floating plunger is described in thesection E. entitled “Force Valve Model.”

[0121] Many different force models may be formulated to predict pressureinduced valve displacement in a pressure environment. Force models mayvary with respect to the type of valve and conditions under which thevalve is intended to operate. The invention is not limited to anyparticular force model.

[0122] As discussed above, the various components of the mass flowcontroller may have a gain term associated with the operation thereof.For example, FIG. 1 illustrates gain terms A, B, C and D associated withthe flow meter 110, the GLL controller 150, the valve actuator 160, andvalve 170, respectively. These components and their associated input andoutput signals, in particular, flow signal FS2, drive signal DS, valvesignal AD, and the fluid flowing in the flow path 103, form a controlloop of the mass flow controller. The gains A, B, C, and D, in turn, areassociated with the relationship between said inputs and outputs. Itshould be appreciated that the gain terms around this control loopcontribute to a composite control loop gain.

[0123] Typically, this control loop gain term is the product of the gainterms around the control loop (i.e., the control loop gain term is equalto the product A*B*C*D). As used herein, a composite gain term describesany gain term comprising the contributions of a plurality of individualgain terms. The notation for a composite gain term used herein will beappear as the concatenation of the symbols used to represent theindividual gain terms contributing to the composite gain term. Forexample, the control loop gain term describe above will be representedas gain term ABCD. Unless otherwise noted, the notation described abovefor a composite gain term is assumed to be the product of itsconstituent gain terms.

[0124] The individual gain terms associated with a control loop of amass flow controller may have differing characteristics and dependenciesresulting in a composite gain term that may have multiple dependencies.These dependencies or variables may include set point or flow rate,fluid species, temperature, inlet and/or outlet pressure, valvedisplacement, etc. Applicants have recognized and appreciated that amass flow controller having an arbitrary control loop gain term may bevulnerable to instability and may be sensitive to changes in some or allof the dependencies mentioned above. Below is a description of each ofthe exemplary gain terms illustrated in FIG. 1.

[0125] Gain term A is associated with the flow meter and represents therelationship between the actual fluid flow through the mass flowcontroller and the indicated flow (e.g., FS2) of the flow meter (e.g.,change in indicated flow divided by change in actual fluid flow). Gainterm A is calibrated to be a constant function of at least flow rate.However, this constant may depend at least upon the fluid species withwhich the mass flow controller operates.

[0126] Gain term B is associated with the GLL controller and representsthe relationship between the indicated flow signal FS2 received from theflow meter and the drive signal DS provided to the valve actuator. Gainterm B is related to the various gains and constants used in thefeedback control of the GLL controller.

[0127] Gain term C is associated with the valve actuator and representsthe relationship between a drive signal and the displacement of thevalve. Gain C may include the combination of two separate gainsincluding the gain associated with the conversion of a drive signal toan electrical current or voltage control signal, and the gain associatedwith the control signal and the mechanical displacement of thecontrolled portion of the valve.

[0128] Gain term D is associated with the valve and represents therelationship between a flow rate of the mass flow controller and valvedisplacement (e.g., a change in flow rate divided by a change in valvedisplacement.) Gain term D may be dependent on a variety of operatingconditions including fluid species, inlet and outlet pressure,temperature, valve displacement, etc. According to one aspect of thepresent invention described in more detail below, a physical model of avalve is provided that facilitates the determination of a gain termassociated with the valve with arbitrary fluids and operatingconditions.

[0129] Gain term G is a reciprocal gain term formed from the reciprocalof the product of gain terms A, C, and D. As will be appreciated furtherfrom the discussion herein, gain term G permits the mass flow controllerto operate in a consistent manner irrespective of operating conditionsby providing to a control loop of the mass flow controller a constantloop gain.

[0130] According to one aspect of the present invention, a system gainterm is determined for a particular mass flow controller by determiningthe composite gain term of various components around the control loop ofthe mass flow controller. A reciprocal gain term is formed by taking thereciprocal of the system gain term. This reciprocal gain term is thenapplied to the control loop such that the control loop operates with aconstant loop gain. Thus, as the various gain terms around the controlloop vary, the reciprocal gain term may be varied in order to maintain aconstant loop gain.

[0131] Because the loop gain of the mass flow controller is heldconstant irrespective of the type of fluid used with the mass flowcontroller, and irrespective of the operating conditions with which themass flow controller is operated, the response of the mass flowcontroller with different fluids and/or operating conditions can be madestable and to exhibit the same behavior as that observed duringproduction of the mass flow controller on a test fluid and testoperating conditions.

[0132] Unless otherwise noted, the system gain term is the composite ofgain terms around the control loop associated with various components ofthe mass flow controller that inherently vary as a function of one ormore operating conditions. For example, the system gain term in FIG. 1is composite gain term ACD.

[0133] In block 140 of FIG. 1, a reciprocal gain term G is formed bytaking the reciprocal of system gain term ACD and applying it as one ofthe inputs to the GLL controller. It should be appreciated that thereciprocal gain term may be the reciprocal of fewer than all of the gainterms associated with the various components around the control loop ofthe mass flow controller. For example, improvements in control andstability may be achieved by forming the reciprocal of composite gainterms AC, AD, CD etc. However, in preferred embodiments, gain term G isformed such that the loop gain remains a constant (i.e., gain G is thereciprocal of the system gain term).

[0134] According to one aspect of the invention, pressure may be sensedat the inlet, and a pressure signal (e.g., pressure signal 190) may beproduced that can be used in association with a mass flow controller.For example, a pressure signal may be produced that can be used in aflow sensor portion of the mass flow controller to compensate forspurious indications due to pressure transients. Further, the pressuresignal may be used for feed forward control of the valve. Also, thepressure signal may be used to adjust the gain in a GLL controller.

[0135]FIG. 2 illustrates a more detailed schematic block diagram of theflow meter 110. A flow meter refers generally to any of variouscomponents that sense flow rate through a flow path, or a portion of aflow path, and provide a signal indicative of the flow rate. The flowmeter 110 of FIG. 2 includes a bypass 210, a sensor and sensorelectronics 230, a normalization circuit 240 to receive the sensorsignal FS 1 from the sensor and sensor electronics 230, a responsecompensation circuit 250 coupled to the normalization circuit 240, and alinearization circuit 260 coupled to the response compensation circuit250. The output of linearization 260 is the flow signal FS2 asillustrated in the mass flow controller of FIG. 1.

[0136] Although not shown in FIG. 2, in some embodiments, the sensorsignal FS1 may be converted to a digital signal with the use of ananalog to digital (A/D) converter so that all further signal processingof the mass flow controller 100 may be performed by a digital computeror digital signal processor (DSP). Although in one preferred embodiment,all signal processing performed by the mass flow controller 100 isperformed digitally, the present invention is not so limited, as analogprocessing techniques may alternatively be used.

[0137] In FIG. 2, a sensor conduit 220 diverts some portion of the fluidflowing through the flow path, with the remainder and majority of thefluid flowing through the bypass. Sensor and sensor electronics 230 arecoupled to the sensor conduit and measure the flow rate through theconduit. A pressure transducer 295 is coupled to flow path 200 upstreamof the bypass to measure the inlet pressure at the inlet side of theflow path 200. Pressure transducer 295 provides a pressure signal 270indicative of the inlet pressure.

[0138] As discussed in the foregoing, pressure transients may causelocal fluctuations in the fluid flow that is sensed by sensor and sensorelectronics 230. However, this is considered false flow information asit is not indicative of the flow rate provided to the process at theoutlet side of the flow path. As such, flow signal FS0 may be corruptedwith false flow information resulting from transients in the inletpressure. For example, flow signal FS0 may contain false flowinformation resulting from local fluid flow fluctuation caused by fluidrushing to fill a dead volume caused by a pressure pulse or otherpressure transient.

[0139] In order to mitigate the effects of the false flow information,compensation filter 280 receives pressure signal 270 from pressuretransducer 295 and constructs false flow signal 290. False flow signal290 is constructed to model the erroneous response of sensor and sensorelectronics 230 due to fluid flow fluctuations caused by pressuretransients. That is to say, false flow signal 290 is constructed toequal or closely approximate the false flow information superimposed onthe flow signal as a result of pressure transients. One suitablecompensation filter was described in detail with respect to FIGS. 8 and12. The false flow signal 290 is then subtracted off flow signal FS0(e.g., by subtractor 297) to provide sensor signal FS1 having the falseflow information effectively removed.

[0140] Sensor signal FS1 is then further processed in order to provideindicated flow signal FS2. In particular, the amount of fluid flowingthrough the conduit is proportional to the fluid flowing in the bypass.However, within the range of flow rates with which a mass flowcontroller is intended to operate, the relationship between the flowrate in the conduit and the flow rate in the bypass may not be linear.

[0141] In addition, thermal sensors measure flow rate by detectingtemperature changes across an interval of the conduit. Accordingly, insome embodiments, particularly those that implement thermal sensors,there may exist temperature dependencies, particularly at the twoextremes of the range of flow rates with which a mass flow controlleroperates (referred to herein as zero flow and full scale flow,respectively).

[0142] Normalization circuit 240 receives the sensor signal FS1 andcorrects for potential temperature dependence at zero flow and at fullscale flow. In particular, when no fluid is flowing through the conduitand/or bypass (i.e., zero flow), the sensor may produce a non-zerosensor signal. Furthermore, this spurious indication of flow may dependon temperature. Similarly, the sensor signal FS1 may experiencefluctuation that is dependent on temperature at full-scale flow.Correction for temperature dependent variation in the signal FS1 at zeroflow may be performed by measuring the value of the sensor signal FS1 atzero flow at a number of different temperatures, and then applying acorrection factor to the signal FS1 based upon the temperature of thesensor. Corrections for temperature dependent variation of sensor signalFS1 at full-scale flow may be performed in a similar manner based uponmeasurements of the sensor signal at different values of temperature andapplying an appropriate correction factor based on the temperature.

[0143] In addition, temperature dependencies may be similarly measuredfor characteristic points along the entire range at which a mass flowcontroller is desired to operate. Accordingly, a correction curve thatis a function of flow rate and temperature may be fit to themeasurements taken a zero flow, full scale flow, and any number ofcharacteristic points in between. This correction curve may providecorrection for temperature dependencies across the range of flow ratewith which the mass flow controller is intended to operate. In addition,a knowledge of the fluid being used and known sensor property variationswith temperature may be utilized to provide or enhance the correctionfactors and/or correction curves of normalization 240.

[0144] The normalization circuit 240 may also provide a fixednormalization gain to the signal FS1 so that at full scale flow throughthe sensor conduit, a specific value is obtained for normalized signalFS1′, and at zero flow, another specific value (e.g. zero) is obtained.

[0145] In one embodiment, for example, normalization 240 ensures that atzero flow through the sensor conduit, normalized signal FS1′ has a valueof 0.0, and at full scale flow through the conduit, normalized signalFS1′ has a value of 1.0. It should be appreciated that any value may bechosen for normalized signal FS1′ at zero flow and at full scale flow,as values used herein are exemplary only.

[0146] It should be appreciated that normalized signal FS1′ may havepoor dynamic characteristics, such that in response to a step change influid flow, the signal FS1′ is delayed in time and smoothed relative tothe actual flow through the flow sensor. This is because thermal flowsensors typically have a slow response time as the thermal changes takeplace over a relatively long period of time.

[0147]FIG. 3 is an illustration of this behavior in which time isplotted on the horizontal or X-axis and flow is plotted on the verticalor Y-axis. As shown in FIG. 3, in response to a unit step change inactual flow through the thermal mass flow sensor, the signal FS1provided by the sensor is delayed in time and smoothed.

[0148] In order to correct for these sensor effects and provide betterdynamic response to changes in fluid flow, normalized signal FS1′ isprovided to response compensation circuit 250. The response compensationcircuit 250 is functionally a filter that is approximately an inverse ofthe transfer function of the sensor and sensor electronics 230. Theresponse compensation circuit 250 may be adjusted or tuned so that theconditioned signal FS1″ provided by the response compensation circuit250 has a predetermined rise time, has a predetermined maximum level ofovershoot and/or undershoot, and levels out within a predetermined timeframe, and/or is tuned for other characteristics that may be desirablefor a particular implementation of a mass flow controller.

[0149] As shown in FIG. 3, the compensated signal FS1″ has a profilethat more closely reflects the profile of the step change in fluid flowthrough the sensor illustrated in the drawing. The flow meter of themass flow controller may be adjusted to provide such a compensatedsignal during production of the mass flow controller. In particular, thedynamic response may be tuned during a sensor tuning step discussed indetail further below.

[0150] As discussed briefly above, the proportion of fluid flowingthrough the sensor conduit relative to the fluid flowing through thebypass may be dependent upon the flow rate of the fluid. In addition,non-linearities in the sensor and sensor electronics further complicatethe relationship between actual fluid flow and the sensed flow signalprovided by the sensor at different flow rates. The result is that acurve representing sensed flow versus fluid flow may not be linear.

[0151] It should be appreciated that many of these non-linearities carrythrough normalization 240 and response compensation 250. Accordingly,the immediate discussion is germane to any of sensor signals FS1, FS1′,and FS1″. The term sensor output will be used herein to describe thesensor signal before it has been linearized (i.e., precedinglinearization 260.) In particular, and unless otherwise indicated,sensor output describes the signal produced by the sensor and that hasbeen normalized and compensated (e.g., FS1″), for example, bynormalization 240 and response compensation 250, respectively, but thathas not been linearized. It should also be appreciated thatnormalization and compensation steps need not respect the order in whichthey are applied in FIG. 2, and are in fact interchangeable.

[0152] Linearization 260 corrects for the non-linearities of the sensoroutput (i.e., FS1″). For example, linearization 260 provides a flowsignal that will have a value of 0 at zero flow, 0.25 at 25% of fullscale flow, 0.5 at 50% of full scale flow, 1.0 at full scale flow etc.Linearization 260 provides the flow signal FS2 provided to an input ofGLL controller 150 as illustrated in FIG. 1. The term indicated flowwill be used herein to describe generally the flow signal provided by aflow meter after it has been linearized (e.g. flow signal FS2).

[0153] Although there are a number of ways to linearize the sensoroutput, such as polynomial linearization, piece-wise linearapproximation, etc., in one embodiment of the present invention, aspline is used to linearize this signal, and in particular, a cubicspline. A discussion of cubic splines is given in Silverman B. W.entitled “Some Aspects of the Spline Smoothing Approach toNon-Parametric regression Curve Fitting”, published in the Journal ofthe Royal Statistics Society and is herein incorporated by reference inits entirety.

[0154] According to this aspect of the present invention, the actualoutput signal FS 1 from the sensor and sensor electronics 230 ismeasured at a number of different (and known) flow rates on a test fluidor gas, and the measured flow rate is plotted against the known flowrate for all measurement points. This plotting of the measured flow rateversus the known flow rate defines the transfer function of the sensorand sensor electronics 230, and a cubic spline is then fit to theinverse of the transfer function of the sensor and sensor electronics230. The measured value of the sensor output is then used as an input tothe cubic spline to provide a normalized, compensated, and linearizedindicated flow signal (e.g., FS2).

[0155] As will be discussed in further detail below, the linearizationcircuit 260 may include a linearization table (not shown) to facilitatelinearization of the sensor output. In an alternative embodiment of thepresent invention, a cubic spline is fit to the transfer function of thesensor and sensor electronics 230 itself, rather than its inverse.

[0156] After compensating for non-linearities in the sensor and sensorelectronics 230, and for the changing fraction of fluid flow that goesthrough the sensor conduit 220, the conditioned flow signal FS2 isprovided to the GLL controller 150 and may also be provided to a filter120 (FIG. 1) for display. An illustration of the conditioned flow signalFS2 is referenced “conditioned sensed flow (FS2)” and shown in FIG. 3.

[0157] As shown in FIG. 1, a gain term A is associated with the flowmeter 110. This gain term represents the relationship between the fluidflowing in the flow path 103 and the indicated flow (i.e., flow signalFS2). In particular, gain term A is the ratio of change in indicatedflow to change in actual fluid flow. It should be appreciated from thediscussion of the flow meter 110 above, that this relationship (i.e., acurve of fluid flow versus indicated flow) has been made to be linear.Thus, the ratio of change in indicated flow to change in actual fluidflow (i.e., the derivative of the curve of fluid flow versus indicatedflow) is a constant function of flow rate. Thus, gain term A is aconstant for a particular fluid species.

[0158] Since gain A is a constant, and since indicated flow has beendefined at a particular value at full scale flow, gain A can bedetermined for a particular fluid based upon the full scale flowassociated with the fluid used during production of the mass flowcontroller. In the exemplary flow meter where indicated flow has beenadjusted to have a value of 1.0 at full scale flow, gain A is simply thereciprocal of full scale flow.

[0159] It should be appreciated that full scale flow through a mass flowcontroller may change as a result of operating the mass flow controllerwith a different fluid. Hence, the mass flow controller will have a fullscale range dependent on fluid species. Therefore, though gain A is aconstant function of at least flow rate, this constant may change uponoperation of the mass flow controller with a different fluid species.

[0160] However, Applicants have determined how the gain associated withthe flow meter (e.g., gain term A) changes with fluid species. Asdiscussed above, the gain of the flow meter can be directly calculatedfrom full scale range (i.e., the full scale flow of the mass flowcontroller). Thus, determining the full scale range for a process fluidallows for a direct determination of the gain of the flow meter. Thefull scale range of a process fluid may be determined by applying aconversion factor to the full scale range associated with a test fluid.The conversion factor may be derived empirically from measurements withthe particular fluid for which the full scale range is being determined.

[0161]FIG. 4 illustrates details of one embodiment of the GLL controller150. Although controller 150 is described herein as being again/lead/lag (GLL) controller, it should be appreciated that thepresent invention is not so limited. For example, the various aspects ofthe present invention may be used with other types of feedbackcontrollers, such as proportional integral differential (PID)controllers, proportional integral (PI) controllers, integraldifferential (ID) controllers, etc. It should also be appreciated thatnumerous mathematical equivalents to the GLL controller 150 illustratedin FIG. 4 may alternatively be used, as the present invention is notlimited to the specific controller structure illustrated therein.

[0162] The GLL controller 150 receives three input signals: the flowsignal FS2 (also referred to as indicated flow); the set point signalSI2; and the reciprocal gain term G. As noted above, the set pointsignal SI2 may first be passed through a slew-rate limiter or filter 130to prevent instantaneous changes in the set point signal from beingprovided to the GLL controller.

[0163] As noted in the foregoing, the Gain G 140 is a reciprocal gainterm formed by taking the reciprocal of the product of the gain termsassociated with various components around a control loop of the massflow controller (i.e., the reciprocal of the system gain term), asdiscussed in detail herein. Gain G may be applied anywhere along thecontrol loop and is not limited to being applied at the input of thecontroller of a mass flow controller. However, reciprocal gain term Gmay be conveniently applied to the input of the GLL controller asillustrated in FIGS. 1 and 4.

[0164] According to one embodiment of the present invention, gain term Gmay be determined by a microprocessor or digital signal processorassociated with the mass flow controller. The processor may beintegrated into the mass flow controller or may be external, asdiscussed below.

[0165] As shown in FIG. 4, the flow signal FS2 is provided to adifferentiator or D-term circuit 410. Because the circuit 410 is notidentically a differentiator, it is referred to as a “D-term” circuitherein. Indeed, within the D-term circuit 410, the flow signal FS2 isdifferentiated, low pass filtered, and multiplied by a constant and thensummed with the conditioned flow signal FS2. It should be appreciatedthat the present invention is not limited to the particularimplementation of the D-term circuit 410 described herein, as othertypes of differentiator circuits may be used. Functionally, the D termcircuit 410 provides a modified flow signal FS3 that is “sped up”relative to the conditioned signal FS2, thereby constituting the “lead”in the GLL controller 150. The D term circuit 410 also provides damping.As should be appreciated by those skilled in the art, the D-term circuit410 functionally provides a modified flow signal FS3 that is indicativeof how the flow signal is changing and how quickly.

[0166] The modified flow signal FS3 is then provided, along with the setpoint signal SI2 to a subtraction circuit 420 that takes the modifiedflow signal FS3 and the set point signal SI2, and generates an errorsignal E based upon their difference. The error signal E is thenmultiplied by the gain term G (hence the word “gain” in a gain/lag/leadGLL controller) and provided to a proportional gain term 440 and anintegral gain term 450.

[0167] The proportional gain term multiplies the signal EG by a fixedconstant Kp, and then provides the output signal EGKP to a summingcircuit 470. The proportional gain term 440 is used to functionallyprovide a component of the drive signal to move the control valve 170 acertain fixed amount based upon the signal EG, thereby allowing thecontrol valve 170 to make up ground quickly upon a change in the errorsignal E.

[0168] The proportional gain term 440 also provides damping, helping toprevent ringing in the drive signal DS and in the resulting flow. Forexample, as the error signal E decreases, and the output signal from theintegrator 460 is increasing, the value of the error signal E multipliedby K_(P) decreases, as the constant K_(P) is preferably less than unity,thereby decreasing the amount of overshoot that occurs.

[0169] The integral gain term 450 multiplies the signal EG by anotherfixed constant K₁, and then provides the output signal EGK_(i) to aninput of the integrator 460. The integrator 460 integrates the signalEGK_(i) and provides the integrated output to a second input of thesumming circuit 470. Functionally, the output of the integrator 460provides a signal that is indicative of the error signal E over time,and represents how the error signal has changed in the past (hence theword “lag” in a gain/lead/lag GLL controller). Given an error signal E,the integrator 460 starts out at a specific slope, and as the indicatedflow (e.g., FS2) increases (assuming a new and higher set point has beeninput), the error signal E decreases, such that the integrator 460 stopsintegrating, (i.e., slows down how fast it's changing) and the componentof the drive signal output from the integrator 460 stops increasing. Theintegrated output signal EGK_(I) is then summed with the output of theproportional gain term EGKP in summing circuit 470, and the summedoutput signal DS is provided as a drive signal to the valve actuator160.

[0170] In addition, a pedestal (not shown) may be provided to preset theintegrator 460 to a particular value when the controller istransitioning from a zero flow to a controlled flow state. The pedestaldescribes a value that when added to the integrator will provide a drivelevel DS that is just below the drive level necessary to open the valveand permit flow. In this manner, the time that would have been necessaryfor the integrator to ramp up to the pedestal value can be eliminatedand the controller will have an increased response time to transitionsbetween zero flow and controlled flow.

[0171] As shown in FIG. 5, the output of the summing circuit is providedto the valve actuator 160 which generally includes a valve driveelectronics circuit 510 that is coupled to an electromechanical actuator520. Any suitable valve drive electronics circuit 510 may be used toreceive the drive signal DS and convert the drive signal DS to avoltage, current, or other signal capable of moving the valve 170 to adesired position to give the desired rate of flow. Further, the valvedrive circuit 510 may include any suitable valve drive actuation circuitknown in the art for driving solenoid actuated control valves,piezoelectrically actuated control valves, etc. According to oneembodiment of the present invention utilizing a solenoid actuatedcontrol valve, the valve drive electronics circuit 510 may includecircuitry that reduces the impact of hysteresis in the solenoid actuatedcontrol valve as described further in detail below.

[0172]FIG. 6 is an illustration of a number of the signals describedabove with respect to FIG. 4 in which the horizontal or X-axisrepresents time and the vertical or Y-axis represents the identifiedsignal level. As shown in FIG. 6A, at a time To, a step change (to thelevel Fo) in the set point in signal SI2 is provided. At this time, theerror signal E rises to the level F₀, as the error signal E is equal tothe difference between the conditioned flow signal FS2 (which is stillat its prior state), and the value of the set point in signal SI2, whichis now at a value of F₀. The error signal times the gain term G (i.e.,signal EG) thus steps to a high value and then decreases with time inthe manner shown in FIG. 6B. As the output of the proportional gain term440 is the signal EG multiplied by the constant K_(P) (which is lessthan unity), the signal EGK_(P) has a similar shape, although slightlyreduced in amplitude, as shown in FIG. 6C. As shown in FIG. 6D, at thetime T₀, the integrated output signal EGK_(I) is zero, but quicklystarts ramping upward due to the magnitude of the error signal E. Theoutput of the summing circuit 470, representing the sum of the outputsignal EGK_(P) and the integrated output signal EGK_(I) is labeled DSand is shown in FIG. 6E. Based upon the drive signal DS provided to thevalve drive and valve drive electronics circuit 160, the control valve170 is opened an increased amount and the indicated flow signal (e.g.,flow signal FS2) starts increasing to the new level of the set point inSI2. As time progresses, the error signal E decreases, the output signalEGK_(P) of the proportional gain term 440 decreases, as does theintegrated output signal EGK_(I), and the rate of flow is established atthe level of the new set point.

[0173] Ideally, it is desired to get a step response in the true flow inresponse to a step input applied to the set point in of the mass flowcontroller. Although this is not practically possible, embodiments ofthe present invention may be used to provide a consistent response inresponse to a step input in the set point, irrespective of whether thestep input represents a 2% step or a 100% step relative to full scaleflow, irrespective of the fluid being used, and irrespective of theinlet or outlet pressure, etc. To obtain this consistency, embodimentsof the present invention provide a mass flow controller having aconstant loop gain.

[0174] It should be appreciated from the foregoing that while variousgains associated with the components along a control loop of a mass flowcontroller may vary as functions of different variables, and may dependupon a variety of different operating conditions, consistent and stableoperation of a mass flow controller can be attained for a set ofoperating conditions by providing the control loop of the mass flowcontroller with a constant loop gain.

[0175] It should be appreciated that various aspects of the control of amass flow controller may be implemented using a microprocessor. Forexample, GLL controller 150 may be implemented as a microprocessor,digital signal processor etc. Likewise, the determination of variouscontrol parameters such as the reciprocal gain term (e.g., gain term G)may be provided by a microprocessor. Various aspects of the control of amass flow controller may be implemented in software, firmware orhardware using techniques that are well known in the art.

[0176] B. Mass Flow Controller Configuration

[0177] It should be appreciated that in many cases, in order for a massflow controller to operate consistently and in a stable manner, the massflow controller must be tuned and/or calibrated during production.Manual tuning and/or calibration is often a time consuming, laborintensive, and expensive process. In addition, when a process requiresthat the mass flow controller be configured to operate with a differentfluid species and/or operating conditions than that used duringproduction, the performance of a mass flow controller will rarelyexhibit the same behavior observed during production of the mass flowcontroller, even if the mass flow controller was tuned and calibrated ona number of process fluids. In other words, the mass flow controller mayhave a different response when operating with a fluid and/or operatingconditions other than that with which the mass flow controller was tunedand/or calibrated.

[0178] According to one aspect of the present invention, a method ofconfiguring a mass flow controller is provided that permits the responseof the mass flow with a process fluid and/or process operatingconditions to be made substantially the same as the response for whichthe mass flow controller was tuned and/or calibrated with a test fluidand test operating conditions.

[0179] In one embodiment of the present invention, during tuning and/orcalibration of a mass flow controller with a single test fluid and a setof test operating conditions, configuration data is obtained. Thisconfiguration data may be used to configure the mass flow controller tooperate with an arbitrary process fluid and/or operating conditions,thus alleviating performance degradation due to operation with a fluidand/or operating conditions other than those used during production, andobviating expensive and time-consuming tuning and/or calibration of themass flow controller on multiple surrogate fluids.

[0180] Providing a mass flow controller that is capable of operatingwith arbitrary fluids and operating conditions and exhibiting asatisfactory response often involves steps including an initialproduction of the mass flow controller and a subsequent configuration ofthe mass flow controller. FIG. 7a illustrates production andconfiguration steps according to one embodiment of the presentinvention.

[0181] The term production, as used herein and when applied to a massflow controller, describes generally the various tasks involved inpreparing a mass flow controller for operation on a specific fluidspecies and a particular set of operating conditions. Production mayinclude building the mass flow controller from various components,operating the mass flow controller on a test fluid under test operatingconditions, and tuning and/or calibrating various components and/orcontrol parameters of the mass flow controller such that the mass flowcontroller exhibits satisfactory behavior and performance (i.e., has asatisfactory response) with the test fluid and test operatingconditions.

[0182] The term configuration or configuring, as used herein and whenapplied to a mass flow controller, describes generally the various stepsinvolved in adapting a mass flow controller for operation with anarbitrary fluid under arbitrary operating conditions. In particular,configuration describes steps involved in adapting a mass flowcontroller for operation with a fluid other than the fluid with whichthe mass flow controller underwent production (referred to herein as a“process fluid” and a “test fluid”, respectively), and under conditionsthat may be different than the set of operating conditions used duringproduction of the mass flow controller (referred to herein as “processoperating conditions” and “test operating conditions”, respectively),such that the response of the mass flow controller is substantially thesame as that observed during production. It should be appreciated thatconfiguration of a mass flow controller may be performed at any timeafter production, and in any location, including, but not limited to,the manufacturing site (e.g., to configure the mass flow controller fora particular known application), or the field (e.g., at an end user'ssite of operation).

[0183] In general, the term satisfactory response refers to a responseof a mass flow controller that performs within a set of given tolerancesof a particular mass flow control process or task. In particular, thedynamic and static response of the mass flow controller performs withina range of tolerances for which the mass flow controller was intended tooperate.

[0184] A mass flow controller may be tuned and/or calibrated duringproduction to have a satisfactory response for an arbitrary set oftolerances. Thus, the response of a mass flow controller after tuningand/or calibration on a test fluid and a set of test operatingconditions, unless otherwise stated, should be considered to have asatisfactory response for that test fluid and operating conditions.However, the response may change substantially when the mass flowcontroller is operated with a different fluid and/or operatingconditions, such that the response is no longer satisfactory.

[0185] In general, a mass flow controller is considered to have the sameresponse on a test fluid and test operating conditions and on a processfluid and/or process operating conditions when both responses aresatisfactory (i.e., both responses perform within the tolerances forwhich the mass flow controller was intended to operate).

[0186] As illustrated in FIG. 7a, during production 710, the mass flowcontroller is operated with a test fluid under a set of test operatingconditions. Characteristics of the operation of the mass flow controllerare obtained and stored as configuration data 712. The configurationdata 712 may be obtained during various tuning and/or calibration stepsof production 710, as described in further detail with respect to FIGS.7b-7 f.

[0187] The term tuning describes steps that involve providingsatisfactory dynamic response and behavior to fluid flow and a change influid flow and/or change in desired fluid flow (i.e., a change in setpoint). The term calibration refers generally to steps that involveproviding a satisfactory steady-state or static response of a mass flowcontroller.

[0188] The term configuration data applies generally to informationobtained during tuning and/or calibration of a mass flow controller. Inparticular, configuration data describes characteristics of and/ormeasurements taken from a mass flow controller during operation with atest fluid and test operating conditions. Configuration data obtainedduring production of a mass flow controller may then be used toconfigure the mass flow controller on a process fluid and/or processoperating conditions.

[0189] As discussed briefly above, the terms test fluid and testoperating conditions are used to describe a fluid and operatingconditions that were used during production of a mass flow controller.The terms process fluid and process operating conditions describe fluidsand operating conditions desired, typically, by an end user for aparticular application of the mass flow controller.

[0190] It should be appreciated that the same type or types of fluidsand operating conditions may be used for both test and process purposes.However, because a mass flow controller cannot be tuned on every fluidand/or under all operating conditions, certain aspects of the inventioninvolve a mass flow controller being tuned and/or calibrated on aparticular test fluid and under a particular set of test operatingconditions during production such that the mass flow controller can beconfigured to operate with a different fluid and/or operating conditionsthereafter. Accordingly, it should be understood that the term “processfluid” is not used to describe different types of fluids, but rather todemonstrate that the fluid may differ from the fluid with which the massflow controller was tuned and/or calibrated. Similarly, the term“process operating conditions” describe a set of operating conditionsthat may not be the same as the test operating conditions with which themass flow controller was tuned and/or calibrated. One, some, or all of aset of process operating conditions may differ from the test operatingconditions.

[0191] In configuration step 720, the configuration data 712 obtainedduring production may be used to facilitate configuration of the massflow controller on a process fluid and/or process operating conditions.According to one embodiment, configuration data 712 is used duringconfiguration 720 to determine control parameters associated with themass flow controller that enable operation of the mass flow controllerwith a process fluid and/or process operating conditions. In particular,the configuration data 712 obtained during a production step 710 is usedto determine control parameters that facilitate the configuration of themass flow controller with a process fluid and process operatingconditions, such that the mass flow controller exhibits a satisfactoryresponse (i.e., the mass flow controller is configured to havesubstantially the same response with the process fluid and/or processoperating conditions as that observed during production using the testfluid and test operating conditions).

[0192] The term control parameter as used herein refers generally toparameters associated with the mass flow controller that facilitate theoperation of the mass flow controller. Control parameters may include,but are not limited to, filter coefficients, gain terms, controllerconstants, linearization curves etc. In particular, control parametersrefer to parameters that may need change, modification, or addition whena mass flow controller is configured for operation with an arbitraryprocess fluid and/or process operating conditions (i.e., configured toexhibit a satisfactory response).

[0193] As used herein, the phrase “configured for operation” is intendedto describe configuring a mass flow controller in such a way that whenoperated, the mass flow controller exhibits a satisfactory response(i.e., mass flow controllers having unsatisfactory responses are notgenerally considered operational).

[0194] It should be appreciated that, in general, production 710 needonly be done once and with a single test fluid and a set of testoperating conditions. However, configuration 720 may be repeated anynumber of times during the lifetime of a mass flow controller. Inparticular, whenever it is desirable to operate the mass flow controllerwith a different process fluid and/or operating conditions, it may bedesirable to repeat configuration 720 with the new process fluid and/orprocess operating conditions such that the mass flow controller exhibitsa satisfactory response with the new process fluid and/or processoperating conditions.

[0195] It should be further appreciated that production andconfiguration of different types of mass flow controllers and differentmass flow controller implementations may require different steps.However, production should include steps such that the mass flowcontroller has been properly characterized and a satisfactory responseestablished for operation with a set of test operating conditions, andthat sufficient configuration data has been obtained to facilitatesubsequent configuration of the mass flow controller. Likewise,configuration in general should include steps necessary to establishsubstantially the same response when operating with a set of processoperating conditions as that observed during production.

[0196]FIG. 7b illustrates a block diagram according to one embodimentthat includes various steps that may be performed during the productionand the configuration (e.g. steps 710 and 720 in FIG. 7a) of a mass flowcontroller. Production 710 may include a sensor tuning step 10, a valvecharacterization step 20, a feedback controller tuning step 30, and acalibration step 40. It should be appreciated that production 710 mayinclude other steps that are not shown in production 710, for example,steps involved with building the mass flow controller, such as bypassmatching etc., that are known in the art.

[0197] In the various exemplary steps 10-40 of production 710, the massflow controller is characterized and a satisfactory response isestablished on a set of test operating conditions. Configuration data isobtained during production that facilitates configuration of the massflow controller for operation with a set of process operatingconditions, as describe further in detail below.

[0198] In sensor tuning step 10, the flow meter of a mass flowcontroller is tuned such that it exhibits a satisfactory dynamicresponse. In particular, the various components of the flow meter aretuned such that the sensor output (e.g. FS1″) responds satisfactorily tochanges in flow through the sensor. For example, as discussed inconnection with FIG. 2, sensor tuning may include providingnormalization and response compensation filter coefficients, correctioncurves, and/or gains such that the flow meter responds to fluid stepswith a sensor output having a step shape that closely resembles the stepchanges in fluid flow in the flow path. In addition, the compensationfilter 280 may be tuned to provide a false flow signal that closelyapproximates the sensor and sensor electronics response to pressuretransients. Information obtained during tuning step 10, such as filtercoefficients, correction curves and/or gain terms may be stored asconfiguration data 712.

[0199] According to on embodiment, the mass flow controller includes atleast one digital filter. This digital filter can be programmed toimplement a compensation filter to compensate for false flow indicationsresulting from pressure transients (e.g., compensation filter 280described in FIGS. 8 and 12).

[0200] In particular, the filter's transform function(s) (e.g.,Equations 6 and/or 7) can be implemented during sensor tuning step 10 byprogramming the digital filter as follows:

J ₀=(2J _(n−1) −J _(n−2))+[(I−J _(n−1))Q−(J _(n−1) −J _(n−2))]P  (3)

[0201] With:

[0202] P=4tξω_(p)/(t²ω_(p) ²+2tξω_(p)+1),

[0203] Q=t(ω_(p)/ξ), and

[0204] t=T_(sample)/2.

[0205] Where:

[0206] ω_(p): Pole frequency. Controls rise/fall time and “width oflobe. Also effects height (gain) of lobe.

[0207] ξ: Damping factor. Controls amount of overshoot. Also effectswidth and height of lobe.

[0208] K: Gain. Sets each of the filter section's portion of thealgorithm output. Effects height of response

[0209] t: Sampling Period T divided by 2.

[0210] The J₀ results from taking the bilinear transform of the filtertransfer function(s). In particular, the transfer function of Equation7. The values of P and Q are calculated such that compensation filterconstructs a desired false flow signal. Parameters ω_(p), ξ, and K,referred to herein as modifiable parameters can be varied in order totune the filter to provide a false flow signal suitable to compensatefor false flow information superimposed on a sensor output signal.

[0211] One method of tuning these parameters during production isdescribed below. During production pressure pulses are introduced to thesensor and a response of the sensor is recorded. In addition, theresponse of a pressure transducer to the pulses is also recorded. Themodifiable parameters are then adjusted to fit the output of the filterto the recorded response of the sensor. For example, a least-squares fitmay be used to minimize the error between the filter output and therecorded sensor response.

[0212] Various optimization methods will occur to those skilled in theart that can be used to adjust filter parameters without departing fromthe scope of the invention. The method described herein is one of methodthat performs a least-squares fit.

[0213] A set of default parameters is chosen for the filter. For thepurpose of this example, the compensation filter to be tuned is similarto that described in connection with FIG. 8. As such, each of the sixsecond order filters will have three modifiable parameters to tune, fora total of eighteen parameters. An exemplary set of default parametersis shown below. Parameter Filter 1 Filter 2 Filter 3 Filter 4 Filter 5Filter 6 K (no units) 0 0 .295 .225 .11 .2 ω_(p) (rad/sec) 600 200 63 6330 2 ξ (no units) 1 1 .56 .79 1 1

[0214] The pressure readings from the pressure transducer correspondingto the pressure pulses introduced to the sensor are input to the defaultcompensation filter to having the default parameters to provide adefault waveform. A matrix W is then generated to store informationrelated to how the default waveform varies with respect to changes inthe modifiable parameters. The matrix W is generated by individuallyvarying the modifiable parameters by some A (e.g., by 1% of the defaultvalues) and generating waveforms from the filter with the variedparameters. Each of these waveforms is then subtracted from the defaultwaveform to produce a difference waveform. As such, 18 differencewaveforms are provided for tuning the filter described in FIG. 8. Thesedifference waveforms are each stored as an entry in the matrix Wproviding a matrix having M×N dimensions where N is the number ofmodifiable parameters and M is the number of samples taken from thefilter output. Therefore the matrix contains information describing eachparameters effect of the filter output.

[0215] As is well known in the art, optimization of 18 parameters may becomputationally expensive. As such, the number of modifiable parametershas been reduced by recognizing the significance of the contribution ofeach parameter. The actual reduction of parameters may vary depending onthe implementation and desired characteristics and control of the filteroutput.

[0216] Filter 1 and 2 are primarily used for the delay they provide. Assuch, the gain terms for these filters may not need to be varied fromtheir default values. Reducing the gain terms to be optimized to K₃, K₄,K₅, and K₆. Filters 1, 2, 5 and 6 will always retain a “damping” factorof 1. As such, only ξ₃ and ξ₄ may need to be optimized. In addition, itmay only be necessary to vary ω₃ and to scale the other frequencyparameters to reserve the relationship shown in the table illustratingexemplary default values. Accordingly, the number of modifiableparameters that need to be optimized is reduced to K₃, K₄, K₅, K₆, ξ₃,ξ₄, ω₃, making the computational task tractable. The resulting matrixaccording to this exemplary reduction of parameters results in a M×7matrix.

[0217] As discussed in the foregoing matrix W describes how the filteroutput changes with respect to changes in the modifiable parameters.From this information a set of changes in the modifiable parameter maybe solved for such that satisfies the expression:

W*ParameterDelta=WaveformError  (Equation 9)

and,

WaveformError=(SensorOutput−DefaultWaveform)  (Equation 10)

[0218] Where:

[0219] SensorOutput=The output from the sensor due to a pressure pulse

[0220] DefaultWaveform=The output of the compensation filter withdefault parameters

[0221] W=The generated difference matrix (M×N)

[0222] ParameterDelta (NX1)=A column vector describing a change in eachof the N modifiable parameters.

[0223] Equation 9 may be true in some best fit sense and may notrepresent absolute equality. ParameterDelta may be solved for accordingto any number of methods that will occur to those skilled in the art.The changes to the N modifiable parameters stored in the ParameterDeltavector are then added to the values of the default parameters to providethe final values of the compensation filter to be stored in the digitalfilter used with the tuned sensor.

[0224] ParameterDelta may be solved for by iteration. As such it may benecessary to update the DefaultWaveform after each iteration and toprovide a CurrentParameter vector to store the accumulatedParameterDelta values. If n is the iteration then,

[0225] CurrentParameter₀=Default Parameters

[0226] CurrentParameter_(n)=CurrentParameter_(n−1)+ParameterDeltan

[0227] DefaultWaveform_(n)=Filter output using the values stored inCurrentParameter_(n)

[0228] WaveformError_(n)=(SensorOutput−DefaultWaveform_(n))

[0229] Various methods of tuning the parameters of a compensation filterwill occur to those skilled in the art. However, the invention is notlimited to the method by which the parameters of the filter areobtained. The various methods and approaches to obtaining a compensationfilter are considered to be in within scope of the invention.

[0230] In valve characterization step 20, the mass flow controller ischaracterized sufficiently such that it can be configured to operate ina consistent and stable manner in response to changes in variousoperating conditions and/or characteristics. According to oneembodiment, a system gain term of a control loop of the mass flowcontroller may be determined and a reciprocal of the system gain termdetermined and applied to the control loop to provide a constant loopgain. In addition, measurements made during the determination of thesystem gain term may be stored as configuration data and later usedduring configuration, as discussed further in detail below with respectto FIG. 7c.

[0231] In feedback controller tuning step 30, the control and controlelectronics associated with the feedback controller are tuned such thatthe mass flow controller exhibits satisfactory dynamic response tochanges in set point. According to one embodiment, the various PIDparameters discussed in connection with FIG. 4 may be set such that theGLL controller exhibits desirable dynamic characteristics such assettling time, maximum overshoot and undershoot, etc.

[0232] In calibration step 40, the mass flow controller is calibratedsuch that it exhibits satisfactory steady-state response. According toone embodiment, the mass flow controller is calibrated to provide alinear relationship between the actual fluid flow through the mass flowcontroller and the flow indicated by the flow meter (e.g. flow signalFS2, also called indicated flow) across the range of flow rates withwhich the mass flow controller was intended to operate.

[0233] In the exemplary steps 50 and 60 illustrated in configuration720, the configuration data obtained during production 710 andinformation about the process operating conditions with which the massflow controller is to be configured for operation is used to modifycontrol parameters of the mass flow controller such that the responseestablished during production does not substantially change whenoperating the mass flow controller with the process operatingconditions.

[0234] As illustrated in FIG. 7b, configuration 720 of the mass flowcontroller may include a system gain decomposition step 50, and a systemconfiguration step 60. In the system gain decomposition step 50, asystem gain term is obtained and then decomposed into its constituentgain terms based, at least in part, on the configuration data obtainedduring production 710 of the mass flow controller.

[0235] However, system gain decomposition step 50 may not be necessaryin some implementations of a mass flow controller and represents onlyone method by which a model of actuator behavior may be provided tosystem configuration step 60.

[0236] Accordingly, it should be appreciated that in the examplesdiscussed herein, steps involving measurement and subsequentdecomposition of a system gain term may be unnecessary undercircumstances where gain terms associated with various components of amass flow controller can be obtained directly. For example, in some massflow controllers, a stepper actuator may be employed from which theassociated gain term may be directly obtained from the mechanical designof the actuator. In such a case, measurement of a system gain duringproduction (e.g. recording CDA′ during valve characterization step 20 inFIG. 7c) and decomposition of the system gain term during configuration(e.g. step 50) can be omitted since the information provided bydecomposing the system gain term (e.g., gain term C) can be obtaineddirectly from the actuator itself.

[0237] The method of obtaining system gain term information duringproduction and decomposing the system gain term during configuration,however, provides a method for configuring a mass flow controller that,in general, may be applied to any implementation of a mass flowcontroller to provide, for instance, a model of the actuator, where noother may be available, or such information cannot be directly obtained.As such, details of this method have been incorporated into productionand configuration steps described in the embodiments illustrated inFIGS. 7c-7 f. However, aspects of the invention are not limited to usingthis method, nor is it limited to mass flow controllers where thismethod may be necessary.

[0238] In the system configuration step 60, control parameters aredetermined for a process fluid and/or process operating conditions forwhich the mass flow controller is being configured, such that the massflow controller exhibits a satisfactory response when operating with theprocess fluid and/or process operating conditions. According to oneembodiment, a reciprocal gain term may be formed from the reciprocal ofthe product of the individual gain terms associated with variouscomponents of the mass flow controller operating with the processoperating conditions. The gain terms may be determined from a physicalmodel of the valve and the valve actuator. The reciprocal gain term maybe applied to a control loop of the mass flow controller to provide aconstant loop gain.

[0239] Further details of exemplary production steps and configurationsteps are now described in connection with FIGS. 7c-7 f.

[0240]FIGS. 7c and 7 d illustrate one exemplary procedure for obtainingconfiguration data during tuning and/or calibration of a mass flowcontroller during production.

[0241]FIGS. 7e and 7 f illustrate another exemplary procedure forconfiguring the mass flow controller to operate on a process fluidand/or process operating conditions different from those with which themass flow controller was tuned and/or calibrated.

[0242] The procedures for production and configuration illustrated inFIGS. 7c-7 f may be applied to a mass flow controller similar to thatillustrated in FIG. 1. However, it should be appreciated that theseaspects of the present invention are not so limited, and may be appliedto a variety of mass flow controllers having a variety of differentcomponents and operating characteristics.

[0243] In FIGS. 7c-7 f, exemplary information that may be stored asconfiguration data during the production of a mass flow controller areillustrated under the heading “Configuration Data” and placed withinblocks labeled 712. It should be appreciated that the informationillustrated in the drawings is not limiting, nor should it be considereda requirement. Each implementation of a mass flow controller may have adifferent set of configuration data that facilitates the configurationof the mass flow controller for operation with a process fluid and/orprocess operating conditions.

[0244]FIG. 7c illustrates further details of a sensor tuning step 10 anda valve characterization step 20 according to one embodiment of thepresent invention. In sensor tuning step 10, the flow meter of a massflow controller is tuned such that it exhibits satisfactory dynamicresponse, for example, to a fluid step. A fluid step refers to a changein fluid flow having the characteristics of a step function, includingboth positive and negative steps in fluid flow.

[0245] In step 12, fluid steps are applied to the flow sensor. The flowsensor is then tuned in step 14, such that in response to a fluid step,a step-shaped flow signal is provided. Desirable characteristics of thisstep-shaped flow signal may include rise time, settling time, maximumovershoot and undershoot, etc. For example, referring back to the massflow controller described with respect to FIGS. 1 and 2, the step oftuning the flow sensor may include tuning of sensor and sensorelectronics 230, normalization 240, and response compensation 250. Forexample, the filter coefficients of the response compensation filter 250may be tuned to reshape the signal as shown in FIG. 3. It should beappreciated that in general, each implementation of a mass flowcontroller may have a different set of parameters that may be tuned.However, the intent of the sensor tuning process 10 is to ensure thatthe flow sensor exhibits satisfactory dynamic characteristics. As shownin FIG. 7c, the normalization gain associated with providing a sensoroutput of 1.0 for full scale flow through the sensor conduit may berecorded as configuration data.

[0246] In the valve characterization process 20, a test fluid isprovided to the mass flow controller at different set points of a set ofselected set points at a known inlet and outlet pressure. At each setpoint the resulting drive level is recorded. The term drive leveldescribes the value of the drive signal provided to the valve actuator.For instance, the drive level may be the measured value of an electricalcurrent or a voltage potential. The drive level may also be the value ofa digital control signal that may be converted into an electrical signalto control the mechanical displacement of the valve. Signal DS in FIG. 1is an example of a drive signal, the value of which is the drive level.

[0247] In one embodiment, a GLL controller that has not been tuned, butthat is known to converge, is used during this step. Accordingly, eachset point in the set of selected set points will converge to the sensoroutput. In some embodiments, the sensor output and drive levelinformation recorded during this step is used to calculate a compositegain term of the mass flow controller. For example, in valvecharacterization step 20 of FIG. 7c, a composite gain term CDA′corresponding to the product of the gain terms associated with the valveactuator 160, the valve 170, and the flow meter 110 is calculated frominformation obtained during the characterization of the valve.

[0248] In step 21, a series of set points from a selected set of setpoints is provided to the mass flow controller. The set of selected setpoints may be chosen in any suitable manner. For example, in oneembodiment, the set of selected set points are various fractions offull-scale flow that account, at some level, for the range with whichthe mass flow controller is intended to operate. The selected set pointsneed not be evenly spaced out across the range of values. In addition,any number of set points may be selected. In general, the number of setpoints selected should be sufficient to adequately characterize thevalve actuator over the range with which the mass flow controller wasintended to operate.

[0249] Each of the various selected sets of set points illustrated inFIGS. 7c-7 f need not be identical to one another. In order toillustrate that the set points need not be the same in each set, thesubscripts vt, cb, and cf, for example, have been used to indicate setpoints chosen for the valve characterization, calibration, andconfiguration steps, respectively. However, it should be appreciatedthat these sets may be in part or entirely the same.

[0250] In step 21, a first set point _(vt)S₀ is chosen from a selectedset of set points {_(vt)S₀, _(vt)S₁, _(vt)S₂, . . . }. A small deviationn is chosen as an offset to the set points _(vt)S_(i). Then, _(vt)S₀+nis applied to the controller and the controller is allowed to converge.When the controller converges, sensor output will equal the applied setpoint. In step 22, the resulting drive level is recorded for set point_(vt)S_(i).

[0251] In step 23, _(vt)S₀−n is applied to the controller and allowed toconverge. The resulting drive level is again recorded as shown in step24. In step 25, a composite gain term CDA′ is determined. For example,the composite gain term may be determined by taking a change in the twoset points (i.e., 2n) and dividing the change by a change in the drivelevels recorded in steps 22 and 24. This ratio represents the compositegain term CDA′ for set point _(vt)S₀. Gain terms C and D, as describedin the foregoing, are associated with the valve actuator and valverespectively. Gain term A′ is associated with the flow meter andrepresents the gain of the flow meter without the contribution oflinearization 260 (i.e., the gain associated with sensor output). Thesensor output value to which the mass flow controller converged for eachset point _(vt)S₁, and the composite gain term CDA′ determined at thatset point may be stored as configuration data.

[0252] Steps 21-25 are repeated for each of the set points _(vt)S_(i) inthe set of selected set points. The result is a set of point pairs{sensor output, CDA′}_(i). In one embodiment, the set of point pairs{sensor output, CDA′}_(i) is recorded as configuration data for themanual tuning of the mass flow controller. In addition, for each CDA′recorded in step 20, a reciprocal gain term G=1/CDA′ may be formed. Thisreciprocal gain term G may be provided to the controller in thesucceeding controller tuning step to provide stability to thecontroller.

[0253] In the feedback controller tuning step 30, the various parametersassociated with the feedback controller of the mass flow controller aretuned to provide satisfactory dynamic response to a series of fluidsteps provided to the mass flow controller. It should be appreciatedthat each implementation of a mass flow controller may have a differentmethod of control (e.g., GLL, PID, ID, etc.). One exemplary procedurefor tuning a feedback controller of a mass flow controller is nowdescribed with respect to the GLL controller depicted in FIG. 4.

[0254] In step 32, the reciprocal gain term G formed from themeasurements made in step 20 is applied to the GLL controller. In step34, fluid steps are provided to the mass flow controller by stepping theset point. For example, SI2 in FIG. 1 is modified by a set of differentchanges in set points ΔS_(i). The different ΔS_(i) may be chosen suchthat the controller is tuned appropriately for large step changes (e.g.,a ΔS_(i) of 100% of full scale flow) and small step changes (e.g., aΔS_(i) of 5% of the full scale flow). The number and magnitude of thevarious ΔS_(i) may differ for each implementation and according to thediffering operating requirements of a particular mass flow controllerimplementation.

[0255] In step 36, the various parameters of the GLL controller are setsuch that the GLL controller responds satisfactorily to the differentchanges in set point as defined by the various ΔS_(i). For example,parameters including the PID constants K_(p), K_(i), etc., may be tunedto provide a desired response to changes in set point. Variouscharacteristics of the controller that may be tuned include, but are notlimited to, rise time, maximum overshoot/undershoot, settling time, etc.

[0256] In calibration step 40, having tuned the sensor and controllerfor a desired dynamic response, and having obtained the composite gainCDA′ for various set points, the mass flow controller undergoes acalibration step to ensure that the mass flow controller has asatisfactory steady-state response. The mass flow controller iscalibrated, in part, such that the relationship between actual fluidflow and indicated flow is linear. In addition, configuration data maybe obtained that facilitates the configuration of the mass flowcontroller on a process fluid and/or process operating conditions asdescribed in calibration step 40 of FIG. 7b.

[0257] In step 41 of calibration step 40, a full scale range is definedfor the mass flow controller. According to one embodiment, the actualfluid flow is measured corresponding to a sensor output of 1.0. Anapproximate linearization curve is provided such that at the definedfull scale flow, indicated flow will have a value at or near 1.0. Theapproximate linearization curve is then applied to the flow meter 110.It should be appreciated that the values of 1.0 for maximum sensoroutput and indicated flow are exemplary and may be replaced with anydesired number.

[0258] In step 43, a first set point _(cb)S₀ is chosen from a set ofselected set points {_(cb)S₀, _(cb)S₁, _(cb)S₂, . . . } and applied tothe mass flow controller. The actual fluid flow in the flow path (e.g.,flow path 103) resulting from the set point is then measured.Corresponding to each set point, the sensor output and actual fluid floware recorded. It should be appreciated that fractional flow (i.e. actualfluid flow divided by the full scale range associated with the testfluid) may be recorded instead of actual fluid flow if more convenient,and that the relevant information is present in both representations.Steps 41 and 43 are then repeated for each of the sets points _(cb)S_(i)in the set of selected set points, resulting in a set of point pairs{sensor output, actual fluid flow}_(i) that may be stored asconfiguration data as illustrated in step 44 and 45.

[0259] The relationship between the point pairs {sensor output, actualfluid flow}_(i) describes the non-linearities associated with the sensorand between the proportion of fluid flowing through the sensor conduitand through the mass flow controller at different flow rates.Accordingly, a linearization curve may be determined from these pointpairs in order to ensure that the relationship between fluid flow andindicated flow is linear. In one embodiment, a set of points thatcorrects for the non-linearities associated with point pairs {sensoroutput, actual fluid flow}_(i) is determined. A cubic spline is fit tothe set of points such that a linearization curve that is continuous andpasses through the point (0,0) (i.e., fluid flow=0 and sensor output=0)is provided. In step 46, the linearization curve is applied to the massflow controller. It should be appreciated that a number of other curvefit methods may alternatively be used, including, but not limited to,piece-wise linear approximation, polynomial approximation, etc.

[0260] During steps 10-40, configuration data has been recorded from thevarious production steps of the mass flow controller on a test fluid andtest operating conditions. The configuration data contains informationthat facilitates configuration of the mass flow controller for operationwith a process gas and/or process operating conditions. It should beappreciated that the set of configuration data recorded during a manualtuning of a mass flow controller may differ depending on the particularimplementation of the mass flow controller, and may differ from thatillustrated in FIGS. 7c and 7 d. Accordingly, configuration data for anyparticular implementation of a mass flow controller merely describesdata obtained during production of a mass flow controller thatfacilitates the configuration of the mass flow controller for operationwith a process fluid and/or process operating conditions.

[0261] For example, in the embodiment illustrated in FIGS. 7c and 7 d,the configuration data recorded during steps 10-40 includes sensortuning parameters, the single gain from the sensor tuning step, tuningconditions, calibration conditions, a set of point pairs {sensor output,CDA′}_(i), a set of point pairs {sensor output, actual fluid flow}_(i),and a full scale range for the test fluid.

[0262] In the valve characterization step 20, the point pairs {sensoroutput, CDA′}_(i) were recorded. As discussed above, the composite gainterm CDA′ is the product of the gain terms associated with the valveactuator, the valve and the flow meter, respectively. However, theindividual contributions of gain terms C, D and A′ to the composite gainterm CDA′ are unknown. Also, it is noteworthy that A′ is only a portionof the total gain term A associated with the flow meter.

[0263] In system gain decomposition 50, the individual gain terms thatcontribute to the composite gain term CDA′ are isolated from thecomposite gain term in order that they may be determined for a processfluid and/or process operating condition in the succeeding systemconfiguration step 60. However, it should be appreciated that steps51-56 may not be necessary for certain implementations of a mass flowcontroller where, for instance, an accurate model of a valve actuator isavailable, or the gain associated with the actuator for a set of processoperating conditions may be directly obtained. As discussed above,system gain decomposition 50 provides a more general method of modelingthe behavior of the valve actuator (e.g., a method of obtaining gainterm C for a set of process operating conditions.)

[0264] In step 51 gain term A is determined. In the previously describedembodiment, the flow meter has been tuned and/or calibrated such that25% of full scale flow results in an indicated flow of 0.25, 50% of fullscale flow results in an indicated flow of 0.5, 75% of full scale flowresults in an indicated flow of 0.75 etc. The relationship between thefluid flow in the flow path and the indicated flow is linear, hence thegain associated with the flow meter (i.e., gain A) is a constant.

[0265] Accordingly, gain A can be directly determined in step 51 bydividing indicated flow by fluid flow at any desired point, the simplestbeing full scale flow and the associated indicated flow of 1 ensured bythe linearization curve. Thus, in embodiments wherein the maximumindicated flow is unity, gain A is equal to the reciprocal of full scalerange (i.e., the value of full scale flow through the mass flowcontroller for a particular fluid species). In general, gain A is equalto the maximum indicated flow value divided by the full scale rangeassociated with a particular fluid species.

[0266] In step 52, composite gain term CDA is formed. Gain term A′ isthe gain associated with the flow meter without the contribution of thelinearization curve while gain term A is a gain associated with the flowmeter including the linearization curve. Therefore, the relationshipbetween A′ and A is by definition the linearization curve. Hence, thecomposite gain term CDA can be directly determined by adding in thecontribution of the linearization curve, which is to say, by multiplyingCDA′ by the gain term associated with the linearization curve (e.g.,multiplying CDA′ by the derivative of the linearization curve). In eachiteration of step 52, gain term CDA′ is formed at set point _(d)S_(i)and provided to step 53.

[0267] In step 53, the contribution of gain term A is removed. Sinceboth the composite gain term CDA and the individual gain term A (thereciprocal of full scale range) are now known, the contribution of gainterm A can be divided out from composite gain term CDA, leavingcomposite gain term CD associated with the valve actuator and the valve.As illustrated in step 53, gain term CD_(i) is formed at set point_(d)S_(i) and provided to step 54.

[0268] As discussed in the foregoing, gain C is the change in valvedisplacement divided by the corresponding change in the drive signal(e.g., DS provided by the GLL controller). Gain D is the change in fluidflow divided by the corresponding change in valve displacement.

[0269] In step 54, gain term D is determined and valve displacement iscalculated at a selected set of set points. In order to furtherdifferentiate composite gain term CD, a physical model of the valve isemployed to determine the valve displacement necessary to achieve aparticular fluid flow under a particular set of operating conditions(i.e., to determine gain D). One physical model of the valve that may beused to make this determination is illustrated and described in SectionD. below, entitled “Physical Valve Model”. It should be appreciated thatdifferent valves and valve types may have different physical models.Furthermore, there may be more than one physical model that may be usedto model the characteristics of any particular valve. Accordingly, thepresent invention is not limited to any particular valve model.

[0270] In one embodiment, gain D is determined by calculating the valvedisplacement necessary to achieve each fluid flow represented by a setof selected set points {_(d)S₀, _(d)S₁, _(d)S₂, . . . }. A deviation nmay be chosen and the gain term D determined by calculating the valvedisplacement at _(d)S_(i)−n and _(d)S_(i)+n and forming the ratio ofchange in set point to change in valve displacement (e.g.,2n/Δdisplacement). Additionally, the displacement at _(d)S_(i) may bedetermined or the values of displacement at _(d)S_(i)−n and _(d)S_(i)+naveraged in order to determine a displacement_(i) at _(d)S_(i). Asillustrated, in each iteration of step 54, gain term D_(i) and thedisplacement_(i) of the valve at set point _(d)S_(i) are determined.

[0271] In step 55, gain term D is divided out of composite gain term CD,thus isolating gain term C. In addition, a set of point pairs {C,displacement}_(i) is generated to provide a model of the behavior of theactuator with the set of test operating conditions used duringproduction 710. It is known that gain term C (the gain associated withthe valve actuator) is not usually directly dependent on process fluidand/or process operating conditions, though it may be a function ofvalve displacement. In each iteration of step 55, the gain term C_(i) isformed by removing the contribution of gain term D_(i) fordisplacement_(i) calculated at set point _(d)S_(i) and stored in the set{C, displacement}_(i)

[0272] Steps 52-55 are repeated for each of the selected set points_(d)S_(i) such that a set of points pairs {C, displacement}_(i) isgenerated that provides information about the behavior of the valveactuator under the set of test operating conditions to the succeedingconfiguration step.

[0273] In system configuration step 60, control parameters aredetermined for a process fluid and/or process operating conditions. Thephysical model considers fluid species, inlet and outlet pressure,temperature, etc. Accordingly, gain D can be calculated for a processfluid and/or process operating conditions by providing the fluid speciesinformation and process operating conditions to the physical model andcalculating the displacements necessary to achieve the variousrepresentative fluid flow values. From the displacements determined fromthe physical model of the valve and model of the behavior of the valveactuator, gain term C may be calculated for the process fluid and/orprocess operating conditions. In one embodiment, the model of thebehavior of the actuator is the point pairs {C, displacement}_(i)generated in system gain decomposition step 50. However, in embodimentswhere the behavior of the valve is known or can be directly measured,gain C can be directly determined from the valve. Thus, having obtainedboth gain terms C and D, the composite gain term CD may be formed.Subsequently, gain A can be calculated by determining a full scale rangefor the process fluid. Accordingly, the system gain term CDA can bedetermined for the process fluid and/or process operating conditions.

[0274] The reciprocal of the system gain term may be formed and appliedto a control loop of a GLL controller (e.g., gain term G). It should beappreciated that G may be a function of one or more operating conditionsof the mass flow controller, such as set point, inlet and/or outletpressure, temperature, etc. Reciprocal gain term G may be applied to theGLL controller such that the control loop of the mass flow controllerhas a constant loop gain with respect to at least the one or moreoperating conditions of which G is a function. Hence, the mass flowcontroller has been configured to operate on a process fluid and/orprocess operating conditions, as discussed further in detail below.

[0275] In step 61, a full scale range associated with a process fluidwith which the mass flow controller is to be configured is determined.One method of determining full scale range is to calculate a conversionfactor based on the specific heat ratios of the process fluid and thetest fluid times the full scale range associated with the test fluid. Itshould be appreciated that other methods may be appropriate forcalculating a full scale range associated with a particular processfluid. For example, the full scale range associated with a particularprocess fluid may be directly measured if appropriate.

[0276] In step 62, gain term D is determined for a process fluid and/orprocess operating conditions from a physical model of the valve byapplying process fluid species information and/or process operatingconditions to the physical model and calculating the displacementnecessary to achieve a set of representative flow values {_(cf)S₀,_(cf)S₁, _(cf)S₂, . . . }. As discussed above, gain D may be determinedby choosing a deviation n and calculating the valve displacement at_(cf)S_(i)−n and _(cf)S_(i)+n and forming the ratio of change in setpoint to change in valve displacement (e.g., 2n/Δdisplacement).Additionally, the displacement at _(cf)S_(i) may be determined or thevalues of displacement at _(cf)S_(i)−n and _(cf)S_(i)+n averaged inorder to determine a displacement_(i) at _(cf)S_(i). Accordingly, ineach iteration of step 62, gain term D_(i) and displacements of thevalve at set point _(cf)S_(i) are determined for the process fluidand/or process operating conditions.

[0277] In step 63, gain term C is determined for a process fluid and/orprocess operating conditions. In some embodiments of the presentinvention gain C may be directly measured from the actuator itself.Alternatively, gain term C may be determined from the information storedin the point pairs {C, displacement}_(i) generated in system gaindecomposition step 50. In either case, in each iteration of step 63,C_(i) is determined at displacement_(i) corresponding to set point_(cf)S_(i) for the process fluid and/or operating conditions.

[0278] In step 64, gain term D is multiplied with gain term C to producecomposite gain term CD. As illustrated, in each iteration of step 64,the product of gain term C_(i) from step 53 and gain term D_(i) fromstep 52 is taken to form composite gain term CD_(i) at set point_(cf)S_(i).

[0279] In step 65, the contribution of gain term A is removed. Sincegain term A is simply the reciprocal of full scale range, composite gainterm CD can be divided by the process full scale range associated withthe process fluid to form system gain term CDA. As illustrated, in eachiteration of step 65, composite gain term CD_(i) is divided by the fullscale range to form system gain term CDA_(i) at set point _(cf)S_(i).

[0280] In step 66, the reciprocal of system gain term CDA is calculatedto form reciprocal gain term G. As illustrated, in each iteration ofstep 66, the reciprocal CDA_(i) is formed and the resulting G_(i) at setpoint _(cf)S_(i) is provided to block 67 to form reciprocal gain term G.It should be appreciated that gain term G may be represented by anynumber of techniques. For example, a curve may be fit to the pointsG_(i), the points G_(i) may be stored in a look-up table, or gain term Gmay be represented in any manner discussed above in connection with thedefinition of a gain term, or otherwise. In addition, gain term G may bea function of one or more operating conditions. In the embodimentillustrated in FIG. 7f, gain term G is a function of set point. However,gain G may additionally be a function of more than one operatingcondition depending on the needs of a particular implementation of amass flow controller.

[0281] Steps 62-66 are repeated for each of the selected set points{_(cf)S₀, _(cf)S₁, _(cf)S₂, . . . } in order to determine reciprocalgain term G for the process fluid and/or process operating conditionswith which the mass flow controller is being configured to operate.

[0282] In step 68 reciprocal gain term G is applied to a control loop ofthe mass flow controller to provide a constant loop gain with respect toat least set point. In general, gain term G will provide a constant loopgain with respect to at least the operating conditions for which it is afunction.

[0283] It should be appreciated that by determining the system gain ofthe mass flow controller based on information for the process fluidand/or process operating conditions, and by applying a reciprocal gainterm of the system gain to a control loop of the mass flow controller,the mass flow controller has been configured for operation with theprocess fluid and/or process operating conditions. In other words, themass flow controller will exhibit the same response observed afterproduction of the mass flow controller with a test fluid and testoperating conditions when operating with the process fluid and/orprocess operating conditions, that is to say, the mass flow controller,when operating with the process fluid and/or process operatingconditions, will exhibit a satisfactory response.

[0284] It should be appreciated that the process of configuring a massflow controller may be automated through the use of a computer. Forexample, steps 50 and 60 may be controlled entirely by a program storedin memory and executed on a processor of a computer, such as a personalcomputer. Hence, a mass flow controller may be automatically configuredfor operation with arbitrary process fluids and/or process operatingconditions.

[0285] The term automatic or automatically as used herein appliesgenerally to a state of being enacted primarily by or under the controlof a computer or processor. In particular, automatic tasks, steps,processes, and/or procedures do not require extensive operatorinvolvement or supervision. Accordingly, an automatic configuration of amass flow controller describes a configuration of a mass flow controllerfor operation with a process fluid and/or process operating conditionsthat does not require manual involvement. Configuration of a mass flowcontroller under the control of a computer program is to be consideredan automatic configuration.

[0286] It should be appreciated that routine tasks such as connecting amass flow controller to a computer or processor, initiating theexecution of a program, etc. are, in general, done manually. However,such tasks are considered routine and may be part of an automaticconfiguration of a mass flow controller.

[0287]FIG. 14 illustrates a system that facilitates automaticconfiguration of a mass flow controller on arbitrary process fluidsand/or process operating conditions. The system includes a mass flowcontroller 1000 and a computer 800.

[0288] The mass flow controller 1000 includes a memory 1002, a processor1004, and the various components of the mass flow controller 1006illustrated and described with respect to FIG. 1. The processor iscoupled to the memory and may be connected to at least some of thecomponents of the mass flow controller. As described above, operation ofa mass flow controller may be implemented under the control of aprocessor, such that the GLL controller 150 is implemented by theprocessor 1004. The mass flow controller 100 further includesconfiguration data 1012 obtained during production of the mass flowcontroller and stored in memory 1002.

[0289] The computer 800 includes a memory 802, a processor 804, an inputdevice, and a program 810 stored in memory 802. The program 810 includesinstructions, that when executed on processor 804, carry out varioussteps involved in configuring a mass flow controller for operation on aprocess fluid and/or process operating conditions (e.g., step 712 inFIG. 7a, steps 60 and 70 in FIGS. 7b, 7 e, and 7 f, etc.).

[0290] It should be appreciated that computer 800 may be any of a numberof computing devices known in the art. For example, computer 800 may bea personal computer, a laptop, a hand held device, or any othercomputing device capable of executing a program. Furthermore, computer800 may be connected to and communicate with the mass flow controller inany number of ways known in the art. For example, computer 800 may beconnected via a cable using any number of standard communication methodsincluding, but not limited to, standard parallel port communication,serial port communication, Universal Serial Bus (USB), etc.Alternatively, the computer 800 may have a wireless connection with themass flow controller. Accordingly, it should be appreciated that thepresent invention is not limited to a particular type of computingdevice, input device, connection type, or communication method, as avariety of types of computing devices, connection types, andcommunication methods may suitably be used.

[0291] According to one embodiment of the present invention, thecomputer 800 may be connected to the mass flow controller in order toconfigure the mass flow controller on a process fluid and/or processoperating condition. The program 810 may then be executed on processor804. Configuration input may be provided to the input device 808. Theconfiguration input may include, but is not limited to, process fluidspecies information, process operating conditions, and/or otherinformation relevant to the configuring of the mass flow controller. Theinput device may be any of a number of devices capable of receivinginformation, including, but not limited to, a keyboard or keypad,interface software for receiving input from a mouse, pointer, etc.

[0292] The program 810 may then obtain configuration data 1012 stored inmemory 1002 of the mass flow controller. From the configuration data andconfiguration input, program 810 determines control parameters for themass flow controller that facilitate operation of the mass flowcontroller with the process fluid and/or process operating conditions.The program 810 may then apply the control parameters to the mass flowcontroller by either modifying existing control parameters accordingly,or by adding additional control parameters to the mass flow controller.In this manner, the mass flow controller may be automatically configuredfor operation with the process fluid and/or process operatingconditions.

[0293] In an alternative embodiment illustrated in FIG. 15, the program810 may be stored in memory 1002 of the mass flow controller and may beexecuted on processor 1004 which may also be used to implement the GLLcontroller 150. An input device 1008 may be added to the mass flowcontroller to enable the mass flow controller to receive configurationinput. Accordingly, the mass flow controller 1000 illustrated in FIG. 15is auto-configurable.

[0294] D. Physical Valve Model

[0295] According to another aspect of the present invention, Applicantshave physically modeled the flow of fluid at different inlet and outletpressures as predominately consisting of two components: the viscouspressure drop and the inviscid (dynamic) pressure drop. By summing thecontributions of each of these components where the effectivedisplacement of the valve for each component is equal, the effectivedisplacement of the valve may be empirically determined using thefollowing methodology. As noted above, the determination of theeffective displacement of the valve at a particular fluid flow rate on aparticular fluid enables the gain term associated with the valve (e.g.,gain term D) to be determined, and thus the determination of the gainterm associated with the valve actuator (e.g., gain term C).

[0296] Referring to FIG. 16, allowing the upstream or inlet pressure tobe represented by P₁ and the downstream or outlet pressure to berepresented by P₂, then at a mass flow rate represented by Q, thevalve-lift is represented by H, and the viscous effect alone reduces thepressure from P₁ to some intermediate pressure P_(x). The inviscidcompressible flow further reduces the pressure from an intermediatepressure P_(x) to P₂. Modeling the viscous pressure drop across thevalve 170 based upon a physical model of viscous flow of fluid betweentwo parallel plates (e.g., between the valve seat and the jet surface),the distance H between the two parallel plates (e.g., the displacementof the valve 170) is provided by the following equation: $\begin{matrix}{H^{3} = {{\frac{{24 \cdot \mu}\quad {QLRT}}{W\left( {P_{1}^{2} - P_{x}^{2}} \right)} \cdot 1.654} \times 10^{- 18}\left( {ft}^{3} \right)}} & \left( {{equation}\quad 1} \right)\end{matrix}$

[0297] where:

[0298] P₁, P_(x): Pressure upstream and downstream of the viscoussurface (psi);

[0299] Q: Mass flow rate (sccm);

[0300] L: length of the flow path (ft);

[0301] H: distance between the two parallel surfaces (ft);

[0302] w: the breadth of the flow path, w equals π·φ, and φ is the meandiameter of plateau 1650, φ is equal to 0.040″ based upon the testedvalve;

[0303] μ: dynamic viscosity of the gas (centi-Poise);

[0304] T: Absolute temperature (deg. Rankine);

[0305] {circumflex over (R)}: universal gas constant, 1545.33(ft-lbf/lb-mole-deg. R); and

[0306] R: gas constant (ft-lbf/lbm-deg. R).

[0307] Modeling the inviscid pressure drop across the valve 170 basedupon a physical model of inviscid flow of fluid through an orifice orjet provides $\begin{matrix}{\frac{Q}{A} = {1.2686 \times 10^{6}{P_{x,0}\left( \frac{2}{\gamma + 1} \right)}^{(\frac{\gamma + 1}{2{({\gamma - 1})}})}\sqrt{\frac{\gamma}{M_{w}T_{1,0}}}}} & \left( {{equation}\quad 2} \right)\end{matrix}$

[0308] for choked flow; and: $\begin{matrix}{\frac{Q}{A} = {1.2686 \times 10^{6}{P_{x,O}\left( \frac{P_{2}}{P_{x,O}} \right)}^{(\frac{\gamma + 1}{2\gamma})}\sqrt{\frac{2\gamma}{\left( {\gamma - 1} \right)M_{w}T_{1,O}}\left\{ {\left( \frac{P_{x,O}}{P_{2}} \right)^{(\frac{\gamma - 1}{`\gamma})} - 1} \right\}}}} & \left( {{equation}\quad 3} \right)\end{matrix}$

[0309] (equation 3)

[0310] for unchoked flow; where the flow is choked if $\begin{matrix}{\frac{P_{2}}{P_{x,0}} \leq \left( \frac{2}{\gamma + 1} \right)^{(\frac{\gamma}{\gamma - 1})}} & \left( {{equation}\quad 4} \right)\end{matrix}$

[0311] and unchoked otherwise, and where

[0312] Q=flow through the valve (sccm);

[0313] A=π·φ·H=valve effective area (sq. in,);

[0314] φ=diameter of orifice 1640;

[0315] M_(w)=gas molecular weight (gm/mol);

[0316] P_(x,o), =upstream total pressure (torr);

[0317] P₂=downstream static pressure (torr);

[0318] T_(1,0)=gas temperature (K);

[0319] γ=ratio of specific heats.

[0320] From the above viscous and inviscid equations, the effectivedisplacement (i.e., H) of the valve 170 may be readily determined.Although some of the units used for the above inviscid calculationsappear to be different from those used in the viscous calculation, thereare no generic difference between the equations and the unit conversionfactors were already built into the numerical constants in eachequation.

[0321] To determine the effective displacement of the valve, assumingthe measured mass flow rate to be Q and the measured upstream anddownstream pressure to be P₁ and P₂ respectively, and neglecting thecontribution of the velocity head to the total pressure, a method ofcalculating the effective displacement of the valve 170 may beperformed. One exemplary method of calculating the effectivedisplacement is to estimate the intermediate pressure Px bytrial-and-error, where one calculates the values of H from both theviscous flow theory (Hv, Eq. 1) and the inviscid theory (Hi, Eq. 2 or3), depending on whether the flow is choked or not, (Eq. 4). Thus, ifthe intermediate pressure is approximately twice the outlet pressure,choked flow may be assumed, and equation 2 is used for the inviscidcomponent of the calculation, whereas if the inlet pressure is less thanapproximately twice the outlet pressure, equation 3 is used for theinviscid component of the calculation. For a given Q, P1, and P2, thecorrect Px is obtained when Hv and Hi become equal to each other. Thus,the computational scheme involves successive iteration to obtain P_(x).The calculation begins by choosing P_(x) to be mid-way between P₁ andP₂. Then the viscous valve-lift (Hv) and the inviscid valve-lift (Hi)are calculated. If it is determined that Hv is greater than Hi, meaningthat there is not enough differential pressure for the viscous flow todeliver the required flow than for the inviscid flow, then during thenext iteration a somewhat lower pressure P_(x)′ will be chosen, i.e.,between the downstream pressure P₂ and the previous pressure P_(x). Theiteration continues until the two calculated valve-lift Hv and Hi comewithin 0.1% of each other. According to a further aspect of the presentinvention, this iterative process may be performed in software. Thesoftware for performing this iterative calculation may readily beperformed by one of ordinary skill in the art and implemented on acomputer. Accordingly, based upon the above method, the effectivedisplacement of the valve 170 may be determined for each of a number ofdifferent flow rates.

[0322] As discussed previously, based upon empirical testing with avariety of different fluids or gases, Applicants have determined how thefractional contribution of the gain A of the mass flow meter changesfrom one gas to another, as it is primarily dominated by the specificheat of the fluid or gas being used. Accordingly, once the mass flowcontroller 100 has been calibrated with a known fluid or gas, how thisgain changes for other types of gases is known. Further, the fractionalcontribution of the gain B of the GLL controller 150 is known to themass flow controller 100, as the various constants that determine thisgain may be stored in a memory of the mass flow controller 100, and thefractional contribution of the gain C of the valve actuator 160 iseffectively constant or known. Accordingly, what remains is a way ofdetermining how the fractional contribution of the gain D of the valve170 and gas path changes for different gases and for different operatingconditions, and how to compensate for changes in the range of the massflow controller 100 for a different fluid or gas than that with whichthe mass flow controller 100 was initially calibrated.

[0323] According to a further aspect of the present invention, a methodof configuring a mass flow controller that has been tuned at under knownconditions and with a known fluid or gas is provided that may be used totune the mass flow controller to have a nearly identical response on adifferent fluid or gas, or with a different operating range that thatwith which it was tuned. As discussed above, mass flow controller 100 isinitially tuned on a known gas (for example, Nitrogen) with a knowninlet pressure and a known outlet pressure. For simplicity, oneembodiment of the present invention selects the known inlet pressure tobe greater than two atmospheres and the outlet pressure at ambient. Thisselection of inlet and outlet pressure is advantageous for two reasons.First, use of inlet and outlet pressures relating to choked flowfacilitate the physical modeling of the valve and valve gas path, asonly choked flow conditions can be used for the inviscid pressure dropequations. Second, this type of operation (i.e., a pressure drop ofapproximately two atmospheres) is typical of the type of operation usedby end-users. Under these conditions, the gain of the gas path may bedefined as: $\begin{matrix}{{gain} = \frac{\left( {{change}\quad {of}\quad {gas}{\quad \quad}{flow}} \right)\text{/}\left( {{full}\quad {scale}\quad {flow}\quad {range}} \right)}{\left( {{change}\quad {of}\quad {valve}\quad {drive}} \right)\text{/}\left( {{Max}\quad {valve}\quad {drive}} \right)}} & \left( {{Equation}\quad 5} \right)\end{matrix}$

[0324] To operate this same mass flow controller on gas “x” with a newfull-scale flow range, the closed-loop gain of the mass flow controller100 may be expected to change as follows: $\begin{matrix}{\frac{{new}\quad {gain}\quad {on}\quad {gas}\quad x}{{old}\quad {gain}\quad {on}\quad N_{2}} = {\left( \frac{1}{{Cfc}_{x}} \right)^{0.4}\left( \frac{{Mw}_{N2}}{{Mw}_{x}} \right)^{0.2}\left( \frac{{old}\quad N_{2}\quad {range}}{{new}\quad N_{2}\quad {range}} \right)}} & \left( {{equation}\quad 6} \right)\end{matrix}$

[0325] where

[0326] Cƒc_(x)=conversion factor “C” for gas x

[0327] Mw=molecular weight of gas

[0328] The above equation is approximate, as there is an additional termwhich is a function of inlet pressure, temperature, and the ratio ofspecific heats. However, the effect of this additional term is to the0.4 power and can normally be neglected. For example, assuming that thecalibration of the mass flow controller 100 was initially performed withNitrogen as the known fluid or gas, the value of this additional termranges from 0.684 for Nitrogen and other diatomic gases, up to 0.726 formonatomic gases, and down to 0.628 for polyatomic gases, then raised tothe 0.4 power. Thus, the difference from Nitrogen is at most about 3.5%and may ordinarily be neglected. To compensate for the above change ingain with a different gas and or different operating conditions thanthose used in calibration the gain term G may be changed by the inverseof the above ratio to provide a constant closed-loop gain for the massflow controller, irrespective of set point, irrespective of operatingconditions, and irrespective of the type of fluid or gas that is used.That is, if the closed-loop gain of the mass flow controller is A*B*C*D,then the gain term G is set to a constant time 1/(A*C*D) to provide aconstant closed-loop gain that is the same as that used duringcalibration.

[0329] E. Force Valve Model

[0330] One suitable force model will be described in connection with avalve using a free floating plunger as illustrated in FIG. 10. Theposition of the plunger # which is controlled by a balance of severalforces. The first force is a spring force that attempts to restore theplunger to its reset position. A second force is a magnetic force fromthe solenoid which attempts to move the plunger away from its restposition, under control of the electronics. A third force is a pressuredifferential between the back of the plunger and the face of theplunger, over the jet orifice and plateau, that attempts to force theplunger toward (for forward flow valves) or away from (for reversed flowvalves) the jet. A fourth force is a flow dependent pressuredifferential between the back of the plunger and the face of the plungeroutside the jet plateau area. This effect can be adequately controlledby the jet design.

[0331] The magnetic force on the plunger depends on the valve mechanics(structure and materials), the valve drive current, and the valvedisplacement. At zero pressure drop, the relationship between drivecurrent and displacement can be calculated. This can be done byutilizing a magnetic model of a nominal valve. It should be appreciatedthat the relationship between drive current and displacement could alsobe calculated from valve gain measurements at specified fluid flows, orit could be measured directly by a laser interferometer peering upthrough the jet.

[0332] At any given displacement and drive current, the derivative ofmagnetic force with respect to drive current, dF/dL, can be calculated.This can calculated from a magnetic model of a nominal valve.

[0333] Fg(p) is the force exerted on the plunger by a pressure drop p

[0334] Fm(d, 1) is the force exerted on the plunger by valve drive d atlift 1

[0335] Fs(1) is the force exerted on the plunger by the spring at lift 1

[0336] L=valve lift

[0337] D=valve drive required at zero pressure drop to provide lift L

[0338] Dd=small change in valve drive

[0339] D′=valve drive required at pressure drop P to provide lift L

[0340] P=pressure drop across the valve

[0341] For a given valve, we know (from a magnetic model of the valve):

[0342] Fm(D, L)

[0343] Fs(L)

[0344] At equilibrium and zero pressure drop, we have:

Fm(D, L)+Fs(L)=0

[0345] This allows us to calculate L(D) at zero pressure drop.

[0346] We wish to have, for any valve lift L:

Fm(D,L)=Fm(D′,L)+Fg(P)

[0347] We will assume that Fm is linear for small Dd:

Fm(D+Dd,L)=Fm(D,L)+Dd*dFm/dD

[0348] This gives us:

Fm(D,L)=Fm(D,L)+Dd*dFm/dD+Fg(P)=>Dd=−Fg(P)/dFm/dD

[0349] Since Fg is proportional to P, we can re-write this as:

Dd=Kp*P/(dFm/dD)

[0350] This allows us to make plunger position independent of P byrunning the valve driver from D′ instead of D:

D′=D+Kp*P/(dFm/dD)  (Equation 11)

[0351] Accordingly, Equation 11 can be used by displacement compensationas described in the foregoing (e.g., displacement compensation asdescribed in FIGS. 9 and 13. In particular, the pressure drop P may bedetermined from pressure measurements in the valve environment. Apressure signal indicative of the pressure drop may be input todisplacement compensation block. The displacement compensation signalmay be related to Kp*P/(dFm/dD). For instance, the displacementcompensation signal may be a drive level necessary to achieve thedisplacement as described in Kp*P/(dFm/dD). This displacementcompensation signal may then be added to a drive signal issued from acontrol loop in order to compensate for pressure induced valvedisplacement.

[0352] For example, a mass flow control valve actuator or driver mayreceive a valve drive signal D from the GLL controller, converts that toa desired current I, then converts that value to a required PWM setting.We need to calculate a corrected valve drive signal D′ as follows:

D′=D+Kp*(Pi−Po)/dF(D)

[0353] where:

[0354] Kp is a valve drive attribute,

[0355] Pi is the inlet pressure,

[0356] Po is an assumed or measured outlet pressure, and

[0357] dF(D) is an arbitrary function of D, dFm/dD evaluated at D.

[0358] Accordingly, a displacement compensation can be implemented tocompensate for valve displacement caused by the pressure gradientbetween the inlet and outlet pressure as seen by the valve.

[0359] The term dF(D) may be fixed for a given controller/valvecombination and it may be possible to determine dF(D) for a particulartype of valve and utilized for each mass flow controller having a valveof that type. As such, dF(D) may be valve dependent and may thereforeneed to be determined for different valve types. One method fordetermining dF(D) is described below.

[0360] A magnetic model of a valve can be used to determine dF(D) for aparticular valve. Magnetic force on the valve plunger is a function ofboth valve drive and lift. Lift, at zero pressure drop, is a function ofboth magnetic force and spring constant, and is thus also a function ofvalve drive.

[0361] Given a valve geometry and spring constant, a finite-elementmagnetic model of a nominal valve can give us force vs. lift curves forvarious valve drive levels. Similarly, the spring constant gives us aspring-force vs. lift line for the spring.

[0362] The intersection of the force vs. lift curve (for a given drivelevel) and the spring-force vs. lift line gives us a nominal lift atthat drive level. The intersections of several force vs. lift curves (atdifferent drive levels) and the spring-force vs. lift line gives usnominal lift as a function of drive, L(D).

[0363] By definition, dF(D) is the derivative of magnetic force on thevalve plunger with respect to valve drive D, given the fixed liftexpected for valve drive D at zero pressure drop.

[0364] For each of several drive levels, we can calculate the nominallift L(D). For each lift, the same finite-element magnetic model of thevalve can give us a force vs. current curve. dF(D) is simply thederivative of the force vs. current curve calculated for L(D), evaluatedat D.

[0365] Matched pairs of D and dF(D) can thus be tabulated for use by thecontroller. For example, dF(D) may be a piecewise approximation to thebehavior of the valve driver, solenoid, and valve. One embodimentinvolves forming a piecewise-liner approximation specified by (D, dF)value pairs. The set of point pairs may be stored in the mass flowcontroller as the magnetic model of the valve. The point pairs may beindexed in order to calculate a displacement compensation signal asdescribed in the foregoing.

[0366] Kp is a valve attribute gain term that may be measured inproduction of a mass flow controller. One method of determining Kpproceeds as follows:

[0367] 1. Select 2 pairs of (inlet pressure, setpoint) meeting thefollowing requirements:

[0368] a. Both pairs require the same valve opening per the combinedviscous/inviscid valve model.

[0369] b. The pressure drop at high inlet pressure is at least 2 times(and preferably 4 or more times) the pressure drop at low inlet.

[0370] c. The valve model is least accurate when both viscous andinviscid models are contributing equally to the result. For both pairs,the flow should be largely determined by the same model (either viscousor inviscid flow). When this is true, the valve model will give anintermediate pressure near the same extreme (either inlet or outlet) inboth cases.

[0371] d. Flow is highly sensitive to valve opening. For viscous flow,this occurs at the highest setpoint. For inviscid flow, this occurs atthe lowest setpoint.

[0372] 2. Set Kp=0 in the controller.

[0373] 3. Cycle the inlet pressure and setpoint between the selectedpairs of values at least 4 (preferably 10) times. Each time, record boththe indicated inlet pressure Pi and valve drive D signals, after flowstabilization, under both high and low inlet pressure.

[0374] 4. Average the recorded values to give:

[0375] Pi1=average indicated inlet pressure under low inlet pressure

[0376] Pi2=average indicated inlet pressure under high inlet pressure

[0377] D1=average valve drive D under low inlet pressure

[0378] D2=average valve drive D under high inlet pressure

[0379] 5. Define:

[0380] Po=average outlet pressure during test, converted to same unitsas Pi1 and Pi2

[0381] 6. Calculate:

Pd1=Pi1−Po

Pd2=Pi2−Po

D0=D1−(D2−D1)*(Pi1−Po)/(Pi2−Pi1)

Kp=((D2−D1)/(P2−P1))/dF(D0)

[0382] As such, K_(p) must be tuned for each unit during production.

[0383] Other improvements and variations may be made according tovarious aspects of the invention. For example, according to one aspectof the invention, feed-forward compensation may be performed on thesystem using pressure information. Because pressure transients (and evenstatic pressure of different value) affect valve operation, a predictionof the effect of pressure on valve operation may be made and compensatedfor. For instance, effects of pressure on the valve may be determined,and a valve drive signal may be compensated for to reduce any inducedvalve motion due to pressure and pressure transients. In one embodiment,a change in valve drive signal can be predicted which is needed tomaintain a plunger of the valve stationary.

[0384] In one embodiment, feed-forward compensation may be performed bycreating a model of the valve to be used, choosing at least two sets offlow rates/pressure states that require the same valve opening, andmeasuring the valve drive signal to produce a calibration value that maybe used to generate parameters for operating the system. In particular,a model of the valve may be created from force vs. displacement vs.drive current curves. Using a physics-based model of the valve itself,at least two flow rates are chosen and at least two correspondingpressure states that require the same valve opening for the modeledvalve. The pressure and set point is cycled between these selected pairsof operating conditions, and the valve drive setting is recorded at eachoperating condition. This valve drive setting provides a calibrationconstant that can be used in conjunction with the valve model togenerate appropriate operational parameters for the device. To obtain asatisfactory measurement, a good electronic pressure controller may beused to cycle the pressure appropriately. Further, there may be sometime and effort needed to develop force vs. displacement vs. drivecurrent curves for each valve configuration used.

[0385] In one embodiment, the calibration constant may be derived bymeasuring the valve pedestal (the current it takes to just barely beginopening the valve) at two different inlet pressures, and making theassumption (albeit false) that the actuator gain is a constant. Thevalve pedestal adjustment, combined with the proper valve model, is asignificant improvement over other compensation methods.

[0386] According to another embodiment of the invention, a dead volumecompensation tuning process may be performed that also uses the pressureinformation (e.g., the pressure signal). More particularly, the pressuresignal may be used to adjust the gain in a GLL controller to provide aconstant gain. It is realized that pressure transients affect thephysical valve model, and therefore, the gain may be adjusted tocompensate for these pressure effects.

[0387] In one embodiment, compensation may be performed using thefollowing process:

[0388] 1. Run a pressure step into the controller. In one example, theinlet pressure may be stepped from approximately 30 PSIG toapproximately 32 PSIG. Other pressures would work as well, but it isrealized that too large a step in pressure provides misleading results.The testing apparatus used to provide the inlet pressure may be modifiedto provide as close to a square-wave step of pressure as possible.

[0389] 2. Record the output of both the pressure transducer and flowsensor during the step.

[0390] 3. Run the recorded pressure transducer output through a model ofthe compensation filter (including the differentiator), and compare theoutput of the transducer with the recorded flow sensor output. Adjustfilter parameters to minimize the difference between the two signals,re-running the model each time. When the difference is within asatisfactory level, the filter adjustment may be stopped, the testconditions recorded, and final filter parameters may be set in thedevice.

[0391] Although the above minimization method may be used, it should beappreciated that any number of minimization methods may be used, and theinvention is not limited to any particular method. For example, onemethod may include using default filter parameters determined fromtypical units, adjusting filter parameters to match up the leading edge,freezing the parameters, adjusting other parameters to match the peak,freezing them, then adjusting the remaining parameters to match thefalling edge. At each step, for example, various linear-least-squaresfits may be used to adjust the parameters. Further, other minimizationalgorithms are equally usable.

[0392] Also, dead volume compensation may be configured for each processgas. In this embodiment, there is a gain adjustment that is part of thedifferentiator (e.g., differentiator 820 of FIG. 8). Differentiator gainis equal to a gain constant divided by the ambient temperature (e.g., inKelvin), and the gain constant may be set (nominally) to the gainrequested by the tuning software described above, multiplied by theambient temperature (in Kelvin) when the tuning data was collected,divided by the conversion factor from the tuning gas to the process gas.

[0393] 1. If the unit is to operate over a wide temperature range,performance may be improved at the ends of that range by selecting anoverall gain proportional to 1/T, where T is the absolute temperature,because the total mass flow for an ideal gas due to a change in pressureis proportional to 1/T, and the sensors generally used are mass flowsensors.

[0394] 2. The gas species affects the gain of the flow sensor. If theunit is to be used on a different gas than the unit was tuned with, theoverall gain needs to be adjusted appropriately. If the gain is notadjusted accordingly, dead-volume compensation can actually makeperformance worse than it would have been without dead volumecompensation.

[0395] 3. The gas species also affects the response of the flow sensor.Performance of the algorithm may be improved if the filter parametersare adjusted as a function of gas species.

[0396] 4. The gain of the flow sensor also varies with flow rate, withgain (typically) decreasing at high flow rates. Performance at high flowlevels may be improved by making the overall gain a function of flowrate. Gain may be related as:

gain=g0+kG*setpoint

[0397] where kG is a small (relative to g0), and is typically negativevalue. Assuming that setpoint is a reasonable analog of flow rate, thisrelation above effectively reduces the gain as a function of flow rate.Alternatively, the actual indicated flow rate may be used instead.

[0398] Further, other improvements may be made, including making gain amore complex function of flow rate, or subtracting the false-flow signalafter linearizing the sensor.

[0399] 5. Response of the flow sensor also varies with flow rate. Forcurrent hardware the change is small, however, so minor improvements inperformance may be made by making filter cascade parameters a functionof either setpoint or flow rate.

[0400] Having described several embodiments of the invention in detail,various modifications and improvements will readily occur to thoseskilled in the art. Such modifications and improvements are intended tobe within the scope of the invention. Accordingly, the foregoingdescription is by way of example only, and is not intended as limiting.The invention is limited only as defined by the following claims and theequivalents thereto.

What is claimed is:
 1. In a flow controller including a flow sensorcoupled to a fluid flow path having an inlet side and an outlet side,the flow sensor being adapted to provide a sensor output signalindicative of a sensed fluid flow through the flow path, a methodcomprising acts of: measuring at least one pressure of the flow path;and adjusting the sensor output signal based on the act of measuring theat least one pressure.
 2. The method of claim 1, further comprising anact of forming at least one pressure signal based on the at least onepressure.
 3. The method of claim 2, further comprising an act offiltering the at least one pressure signal to provide a false flowsignal that emulates a response of the flow sensor due to pressurechanges in the flow path.
 4. The method of claim 3, wherein the act ofadjusting the sensor output includes an act of subtracting the falseflow signal from the sensor output signal.
 5. A method of modifying asensor output signal from a flow sensor, the method comprising acts of:constructing a false flow signal corresponding to a response of the flowsensor due to changes in pressure based on at least one pressuremeasurement of the flow path; and subtracting the false flow signal fromthe sensor output signal.
 6. The method of claim 5, further comprisingan act of providing a pressure signal indicative of the at least onepressure measurement.
 7. The method of claim 6, wherein the act ofconstructing a false flow signal includes an act of delaying thepressure signal such that it is substantially aligned in time with thesensor output signal.
 8. The method of claim 6, wherein the act ofconstructing the false flow signal includes an act of differentiatingthe pressure signal.
 9. The method of claim 6, wherein the act ofconstructing the false flow signal includes an act of filtering thepressure signal with at least one filter, the at least one filter havinga transfer function that emulates a response of the flow sensor to thepressure change in the flow path.
 10. The method of claim 9, wherein theat least one filter includes a plurality of 2^(nd)-order filtersconnected in series, and an output from each of the plurality of2^(nd)-order filters are scaled and summed to provide the false flowsignal.
 11. A method of removing false flow information from a sensoroutput signal provided by a flow sensor coupled to a flow path, thefalse flow information resulting from the flow sensor responding to flowchanges caused by pressure transients, the method comprising acts of:measuring at least one pressure in the flow path; providing at least onepressure signal indicative of the at least one pressure measurement;constructing a false flow signal from the at least one pressure signal;and subtracting the false flow signal from the sensor output signal toprovide a flow signal indicative of the fluid flow in the fluid path.12. A method of dead volume compensation, the method comprising acts of:predicting a response of a sensor to a fluid filling a dead volume dueto pressure changes in a fluid flow path; and modifying a sensor outputsignal provided by the sensor based on the predicted response toessentially remove false flow information from the sensor output signal.13. A method of determining a flow rate of a fluid flowing in a conduit,comprising acts of: a) sensing a flow rate of the fluid flowing in theconduit; b) measuring a change in pressure of the fluid flowing in theconduit; c) determining an effect of the change in pressure on the flowrate of the fluid sensed by act (a); and d) modifying the sensed flowrate of the fluid based upon the effect of the change in pressure todetermine the flow rate of the fluid flowing in the conduit.
 14. A flowmeter comprising: a flow sensor adapted to measure fluid flow in a flowpath, the flow sensor providing a sensor output signal in response tosensed fluid flow in the flow path; at least one pressure transducer tomeasure at least one pressure in the flow path, the at least onepressure transducer providing at least one pressure signal related tothe respective at least one measured pressure; a compensation filter toreceive the at least one pressure signal, the compensation filteradapted to construct a false flow signal approximating a response of theflow sensor to pressure transients in the flow path; and a subtractor toreceive the sensor output signal and the false flow signal and toprovide a flow signal related to the difference between the sensoroutput signal and the false flow signal.
 15. The flow meter of claim 14,wherein the compensation filter includes a delay block that delays theat least one pressure signal to be substantially aligned in time withthe response of the flow sensor to pressure transients, and wherein thedelay block provides at least one delayed pressure signal.
 16. The flowmeter of claim 15, wherein the compensation filter includes adifferentiator to receive the delayed pressure signal, thedifferentiator being adapted to determine a derivative of the delayedpressure signal to provide a derivative signal.
 17. A compensationfilter for generating a false flow signal from a pressure signal, thecompensation filter comprising: a differentiator receiving a pressuresignal indicative of a pressure in a fluid path, the differentiatorbeing adapted to determine a derivative of the pressure signal toprovide a derivative signal; and at least one filter having a transferfunction adapted to transform the derivative signal into a false flowsignal indicative of false flow information generated by the flow sensorin response to pressure transients.
 18. A method of compensating forfluid pressure induced changes in the position of the controlled portionof a valve, the method comprising acts of: measuring at least onepressure in a valve environment; providing at least one pressure signalindicative of the at least one pressure measurement, respectively;calculating a displacement of the controlled portion of the valve basedon the at least one pressure signal; and generating a compensation drivelevel to move the controlled portion of the valve an amount having anopposite sign of and substantially equal in magnitude to the calculateddisplacement.
 19. A method of preventing the movement of the controlledportion of the a valve due to pressure transients, the method comprisingacts of: predicting a displacement a pressure transient will force thecontrolled portion of a valve to move based on at least one pressuremeasurement of a valve environment; and moving the controlled portion ofthe valve to counter-act the predicted displacement.
 20. An apparatuscoupled to a flow path, the apparatus comprising: a pressure measurementdevice to measure at least one pressure in a flow path environment andto provide at least one pressure signal indicative of the at least onemeasured pressure; and displacement compensation means for receiving theat least one pressure signal and for providing a displacementcompensation signal indicating a drive level to compensate for valvedisplacement of a valve coupled to the flow path caused by pressurechanges in the flow path environment.
 21. The apparatus of claim 20,wherein the displacement compensation means comprises means forcalculating the displacement compensation signal based on a force valvemodel.
 22. The apparatus of claim 21, wherein the force valve modelincludes a magnetic model of the valve.
 23. The apparatus of claim 21,wherein the force valve model has a parameter that indicates a pressuregradient in the valve environment.
 24. A flow meter comprising: a flowsensor adapted to sense fluid flow in a fluid flow path and to provide asensor output signal indicative of the sensed fluid flow; at least onepressure transducer adapted to measure at least one pressure in a fluidflow path environment and to provide at least one pressure signalindicative of the at least one measured pressure; and a compensationfilter to receive the at least one pressure signal and to construct afalse flow signal related to the at least one pressure signal.
 25. Theflow meter of claim 24, wherein the false flow signal is constructed torecreate false flow information resulting from the flow sensor responseto flow fluctuations caused by pressure transients in the flow path. 26.The flow meter of claim 24, wherein the compensation filter includes atransfer function that emulates a response of the flow sensor topressure transients in the flow path.
 27. The flow meter of claim 24,wherein the false flow signal is subtracted from the sensor outputsignal to provide a flow signal.
 28. In a mass flow controller coupledto a flow path, the mass flow controller having a control loop includinga flow meter, a controller, a valve actuator and a valve, a methodcomprising acts of: measuring at least one pressure in a fluid pathenvironment; providing at least one pressure signal indicating at leastone pressure measurement; determining at least one compensation signalbased on at least one pressure measurement; and applying the at leastone compensation signal to the control loop of the mass flow controller.29. The method of claim 28, wherein the act of determining at least onecompensation filter includes constructing a false flow signal torecreate false flow information resulting from a response of the flowmeter to pressure transients in the flow path environment.
 30. Themethod of claim 29, wherein the act of applying the at least onecompensation signal to the control loop includes an act of applying thefalse flow signal to the control loop to compensate for the flow metersresponse fluctuations in fluid flow due to pressure transients in theflow path.
 31. The method of claim 27, wherein the act of determiningthe at least one compensation signal includes determining a displacementcompensation signal indicative to a drive level to compensate for avalve displacement due to pressure transients.
 32. The method of claim27, wherein the act of determining the at least one compensation signalincludes determining a false flow signal and a displacement compensationsignal.
 33. A mass flow controller having a control loop, the mass flowcontroller comprising: a flow meter adapted to sense fluid flow in afluid flow path and provide a flow signal indicative of the mass flowrate in the flow path; a controller coupled to the flow meter andadapted to provide a drive signal based at least in part on the flowsignal; a valve actuator adapted to receive the drive signal from thecontroller; a valve adapted to be controlled by the valve actuator andcoupled to the fluid flow path; at least one pressure transducer tomeasure at least one pressure in a mass flow controller environment andto provide at least one pressure signal indicative of measurement of theat least one pressure; and at least one compensation means to receive atleast one pressure signal and to provide at least one compensationsignal to the control loop to compensate for effects of add pressurechanges in the mass flow controller environment, wherein the controlloop of the mass flow controller includes the flow meter, thecontroller, the valve actuator, and the valve.
 34. The mass flowcontroller of claim 33, wherein the at least one transducer measures aninlet pressure of the flow path and provides an inlet pressure signal.35. The mass flow controller of claim 34, wherein the at least onecompensation means includes a compensation filter to receive the inletpressure signal and to construct a false flow signal from the inletpressure signal.
 36. The mass flow controller of claim 35, wherein theflow meter includes a flow sensor adapted to sense fluid flow in theflow path and adapted to provide a sensor output signal indicative ofthe sensed fluid flow.
 37. The mass flow controller of claim 36, whereinthe compensation filter has a transfer function that emulates theresponse of the flow sensor to fluid flow resulting from changes ininlet pressure.
 38. The mass flow controller of claim 36, wherein thefalse flow signal is constructed to recreate a false flow informationcomponent of the sensor output signal resulting from changes in inletpressure.
 39. The mass flow controller of claim 36, wherein the flowsignal is determined by subtracting the false flow signal from thesensor output signal.
 40. The mass flow controller of claim 33, whereinthe compensation means includes displacement compensation means thatreceives the inlet pressure signal and provides a displacementcompensation signal indicative of a drive level to maintain a controlledportion of the valve substantially motionless in a pressure environmentof the valve.
 41. The mass flow controller of claim 40, wherein thedisplacement compensation signal is added to the drive signal tocompensate for valve displacement resulting from pressure gradients inthe pressure environment of the valve.
 42. The mass flow controller ofclaim 40, wherein the displacement compensation signal is based in parton a force model of the valve.
 43. The mass flow controller of claim 42,wherein the force model of the valve includes a magnetic model of thevalve.
 44. The mass flow controller of claim 42, wherein the force modelof the valve includes a parameter for at least one pressure drop acrossthe valve.
 45. The mass flow controller of claim 33, wherein thecompensation means includes a compensation filter receiving at least onepressure signal and providing a false flow signal constructed torecreate false flow information resulting from the flow meter respondingto pressure transients and displacement compensation means to receive atleast one pressure signal and to provide a displacement compensationsignal indicative of a drive level to compensate for valve displacementcaused by a pressure change.
 46. A method of configuring a mass flowcontroller for operation with process operating conditions that differat least in part from test operating conditions used during productionof the mass flow controller, the method comprising acts of: establishinga response of the mass flow controller with the test operatingconditions; and modifying at least one control parameter of the massflow controller based on the process operating conditions such that theresponse of the mass flow controller operating with the processoperating conditions does not substantially change.
 47. The method ofclaim 46, wherein the act of modifying the at least one controlparameter includes an act of determining a plurality of process gainterms associated with a plurality of components of the mass flowcontroller based on the process operating conditions, the plurality ofcomponents forming a control loop of the mass flow controller.
 48. Themethod of claim 47, wherein the act of determining the plurality ofprocess gain terms includes an act of determining a process reciprocalgain term formed by taking a reciprocal of a product of the plurality ofprocess gain terms, the process reciprocal gain term being a function ofat least one variable operating condition.
 49. The method of claim 48,wherein the at least one variable operating condition includes at leastone pressure in the mass flow controller environment.
 50. The method ofclaim 49, wherein the at least one variable operating condition includesan inlet pressure.
 51. The method of claim 49, wherein the at least onevariable operating condition includes a set point.
 52. A computerreadable medium encoded with a program for execution on a processor, theprogram, when executed on the processor performing a method ofconfiguring a mass flow controller for operation with a set of processoperating conditions that differ at least in part from a set of testoperating conditions used to establish a response of the mass flowcontroller during production, the method comprising acts of: receivingas an input at least one of process fluid species information andprocess operating conditions; and modifying at least one controlparameter of the mass flow controller based on the input such that theresponse of the mass flow controller does not substantially change whenoperated with the process operating conditions.
 53. The computerreadable medium of claim 52, wherein that act of modifying the at leastone control parameter includes an act of determining a plurality ofprocess gain terms associated with a plurality of components of the massflow controller operating with the process operating conditions, theplurality of components forming a control loop of the mass flowcontroller.
 54. The computer readable medium of claim 53, wherein theact of determining the plurality of gain terms includes an act ofdetermining a process reciprocal gain term formed by taking a reciprocalof a product of the plurality of gain terms, the process reciprocal gainterm being a function of at least one variable operating condition. 55.The computer readable medium of claim 54, wherein the at least onevariable operating condition includes at least one pressure in the massflow controller environment.
 56. The computer readable medium of claim55, wherein the at least one variable operating condition includes aninlet pressure.
 57. The computer readable medium of claim 55, whereinthe at least one variable operating condition includes a set point. 58.A mass flow controller having a control loop, the mass flow controllercomprising: a flow meter adapted to sense fluid flow in a fluid flowpath and provide a flow signal indicative of the mass flow rate in theflow path; a controller coupled to the flow meter and adapted to providea drive signal based at least in part on the flow signal; a valveactuator adapted to receive the drive signal from the controller; avalve adapted to be controlled by the valve actuator and coupled to thefluid flow path; wherein the control loop of the mass flow controllerincludes the flow meter, the controller, the valve actuator, and thevalve; and wherein the control loop is adapted to have a substantiallyconstant control loop gain term with respect to at least one variableoperating condition during operation.
 59. The mass flow controller ofclaim 58, wherein the at least one variable operating condition includesat least one pressure in the mass flow controller environment.
 60. Themass flow controller of claim 59, wherein the at least one variableoperating condition includes an inlet pressure.
 61. The mass flowcontroller of claim 59, wherein the at least one variable operatingcondition includes a set point.
 62. A compensation filter for generatinga false flow signal from a pressure signal, the compensation filtercomprising: a plurality of filters, at least two of which are connectedin series, and wherein a respective output of each of the at least twofilters are scaled and summed.
 63. The compensation filter according toclaim 62, further comprising a differentiator that is adapted todifferentiate the pressure signal, and which provides a differentiatedsignal to the plurality of filters.
 64. The compensation filteraccording to claim 62, further comprising a delay that delays thepressure signal, and which provides a delayed pressure signal to theplurality of filters.